diff --git a/src/lib/AttributeResolver.php b/src/lib/AttributeResolver.php
index 0063d8df..e8f936cb 100644
--- a/src/lib/AttributeResolver.php
+++ b/src/lib/AttributeResolver.php
@@ -7,8 +7,6 @@
 
 namespace cebe\yii2openapi\lib;
 
-use cebe\yii2openapi\lib\Config;
-use cebe\yii2openapi\lib\CustomSpecAttr;
 use cebe\yii2openapi\lib\exceptions\InvalidDefinitionException;
 use cebe\yii2openapi\lib\items\Attribute;
 use cebe\yii2openapi\lib\items\AttributeRelation;
@@ -22,7 +20,6 @@
 use Yii;
 use yii\helpers\Inflector;
 use yii\helpers\StringHelper;
-use yii\helpers\VarDumper;
 use function explode;
 use function strpos;
 use function strtolower;
@@ -266,7 +263,7 @@ protected function resolveProperty(
 
             $relation = Yii::createObject(
                 AttributeRelation::class,
-                [$property->getName(), $relatedTableName, $relatedClassName]
+                [static::relationName($property->getName(), $property->fkColName), $relatedTableName, $relatedClassName]
             )
                            ->asHasOne([$fkProperty->getName() => $attribute->columnName]);
             $relation->onUpdateFkConstraint = $property->onUpdateFkConstraint;
@@ -317,7 +314,7 @@ protected function resolveProperty(
                     $this->relations[$property->getName()] =
                         Yii::createObject(
                             AttributeRelation::class,
-                            [$property->getName(), $relatedTableName, $relatedClassName]
+                            [static::relationName($property->getName(), $property->fkColName), $relatedTableName, $relatedClassName]
                         )
                            ->asHasMany([$fkProperty->getName() => $fkProperty->getName()])->asSelfReference();
                     return;
@@ -326,7 +323,7 @@ protected function resolveProperty(
                 $this->relations[$property->getName()] =
                     Yii::createObject(
                         AttributeRelation::class,
-                        [$property->getName(), $relatedTableName, $relatedClassName]
+                        [static::relationName($property->getName(), $property->fkColName), $relatedTableName, $relatedClassName]
                     )
                        ->asHasMany([$foreignPk => $this->schema->getPkName()]);
                 return;
@@ -345,7 +342,7 @@ protected function resolveProperty(
             $this->relations[$property->getName()] =
                 Yii::createObject(
                     AttributeRelation::class,
-                    [$property->getName(), $relatedTableName, $relatedClassName]
+                    [static::relationName($property->getName(), $property->fkColName), $relatedTableName, $relatedClassName]
                 )
                    ->asHasMany([Inflector::camel2id($this->schemaName, '_') . '_id' => $this->schema->getPkName()]);
             return;
@@ -492,4 +489,14 @@ protected function resolvePropertyRef(PropertySchema $property, Attribute $attri
         $this->attributes[$property->getName()] =
             $attribute->setFakerStub($this->guessFakerStub($attribute, $fkProperty));
     }
+
+    public static function relationName(string $propertyName, ?string $fkColumnName): string
+    {
+        $fkColumnName = (string) $fkColumnName;
+        $relationName = $propertyName;
+        if (!str_contains($fkColumnName, '_')) {
+            $relationName = strtolower($fkColumnName) === strtolower($relationName) ? $relationName . 'Rel' : $relationName;
+        }
+        return $relationName;
+    }
 }
diff --git a/src/lib/ValidationRulesBuilder.php b/src/lib/ValidationRulesBuilder.php
index 3f3c1398..0c5403f2 100644
--- a/src/lib/ValidationRulesBuilder.php
+++ b/src/lib/ValidationRulesBuilder.php
@@ -10,8 +10,7 @@
 use cebe\yii2openapi\lib\items\Attribute;
 use cebe\yii2openapi\lib\items\DbModel;
 use cebe\yii2openapi\lib\items\ValidationRule;
-use yii\helpers\VarDumper;
-use yii\validators\DateValidator;
+use yii\helpers\Inflector;
 use function count;
 use function implode;
 use function in_array;
@@ -162,10 +161,12 @@ private function addExistRules(array $relations):void
             } elseif ($attribute->phpType === 'string') {
                 $this->addStringRule($attribute);
             }
+
+            $targetRelation = AttributeResolver::relationName(Inflector::variablize($attribute->camelName()), $attribute->fkColName);
             $this->rules[$attribute->columnName . '_exist'] = new ValidationRule(
                 [$attribute->columnName],
                 'exist',
-                ['targetRelation' => $attribute->camelName()]
+                ['targetRelation' => $targetRelation]
             );
         }
     }
diff --git a/tests/specs/blog/models/base/Comment.php b/tests/specs/blog/models/base/Comment.php
index 44cd42fc..1cf49178 100644
--- a/tests/specs/blog/models/base/Comment.php
+++ b/tests/specs/blog/models/base/Comment.php
@@ -28,9 +28,9 @@ public function rules()
             'trim' => [['post_id'], 'trim'],
             'required' => [['post_id', 'author_id', 'message', 'created_at'], 'required'],
             'post_id_string' => [['post_id'], 'string', 'max' => 128],
-            'post_id_exist' => [['post_id'], 'exist', 'targetRelation' => 'Post'],
+            'post_id_exist' => [['post_id'], 'exist', 'targetRelation' => 'post'],
             'author_id_integer' => [['author_id'], 'integer'],
-            'author_id_exist' => [['author_id'], 'exist', 'targetRelation' => 'Author'],
+            'author_id_exist' => [['author_id'], 'exist', 'targetRelation' => 'author'],
             'message_default' => [['message'], 'default', 'value' => []],
             'meta_data_default' => [['meta_data'], 'default', 'value' => []],
             'created_at_integer' => [['created_at'], 'integer'],
diff --git a/tests/specs/blog/models/base/Post.php b/tests/specs/blog/models/base/Post.php
index 457978b5..d1949af2 100644
--- a/tests/specs/blog/models/base/Post.php
+++ b/tests/specs/blog/models/base/Post.php
@@ -30,9 +30,9 @@ public function rules()
             'trim' => [['title', 'slug', 'created_at'], 'trim'],
             'required' => [['title', 'category_id', 'active'], 'required'],
             'category_id_integer' => [['category_id'], 'integer'],
-            'category_id_exist' => [['category_id'], 'exist', 'targetRelation' => 'Category'],
+            'category_id_exist' => [['category_id'], 'exist', 'targetRelation' => 'category'],
             'created_by_id_integer' => [['created_by_id'], 'integer'],
-            'created_by_id_exist' => [['created_by_id'], 'exist', 'targetRelation' => 'CreatedBy'],
+            'created_by_id_exist' => [['created_by_id'], 'exist', 'targetRelation' => 'createdBy'],
             'title_unique' => [['title'], 'unique'],
             'slug_unique' => [['slug'], 'unique'],
             'title_string' => [['title'], 'string', 'max' => 255],
diff --git a/tests/specs/blog_v2/models/base/Comment.php b/tests/specs/blog_v2/models/base/Comment.php
index a732c5c0..0f9fc288 100644
--- a/tests/specs/blog_v2/models/base/Comment.php
+++ b/tests/specs/blog_v2/models/base/Comment.php
@@ -28,9 +28,9 @@ public function rules()
             'trim' => [['message', 'meta_data', 'created_at'], 'trim'],
             'required' => [['post_id', 'message', 'created_at'], 'required'],
             'post_id_integer' => [['post_id'], 'integer'],
-            'post_id_exist' => [['post_id'], 'exist', 'targetRelation' => 'Post'],
+            'post_id_exist' => [['post_id'], 'exist', 'targetRelation' => 'post'],
             'user_id_integer' => [['user_id'], 'integer'],
-            'user_id_exist' => [['user_id'], 'exist', 'targetRelation' => 'User'],
+            'user_id_exist' => [['user_id'], 'exist', 'targetRelation' => 'user'],
             'message_string' => [['message'], 'string'],
             'meta_data_string' => [['meta_data'], 'string', 'min' => 1, 'max' => 300],
             'meta_data_default' => [['meta_data'], 'default', 'value' => ''],
diff --git a/tests/specs/blog_v2/models/base/Post.php b/tests/specs/blog_v2/models/base/Post.php
index 44fe4275..333167e6 100644
--- a/tests/specs/blog_v2/models/base/Post.php
+++ b/tests/specs/blog_v2/models/base/Post.php
@@ -32,9 +32,9 @@ public function rules()
             'trim' => [['title', 'slug', 'created_at'], 'trim'],
             'required' => [['title', 'category_id', 'active'], 'required'],
             'category_id_integer' => [['category_id'], 'integer'],
-            'category_id_exist' => [['category_id'], 'exist', 'targetRelation' => 'Category'],
+            'category_id_exist' => [['category_id'], 'exist', 'targetRelation' => 'category'],
             'created_by_id_integer' => [['created_by_id'], 'integer'],
-            'created_by_id_exist' => [['created_by_id'], 'exist', 'targetRelation' => 'CreatedBy'],
+            'created_by_id_exist' => [['created_by_id'], 'exist', 'targetRelation' => 'createdBy'],
             'title_unique' => [['title'], 'unique'],
             'title_string' => [['title'], 'string', 'max' => 255],
             'slug_string' => [['slug'], 'string', 'min' => 1, 'max' => 200],
diff --git a/tests/specs/fk_col_name/app/models/base/Webhook.php b/tests/specs/fk_col_name/app/models/base/Webhook.php
index 8746e8f0..6b1d0b44 100644
--- a/tests/specs/fk_col_name/app/models/base/Webhook.php
+++ b/tests/specs/fk_col_name/app/models/base/Webhook.php
@@ -25,9 +25,9 @@ public function rules()
         return [
             'trim' => [['name'], 'trim'],
             'user_id_integer' => [['user_id'], 'integer'],
-            'user_id_exist' => [['user_id'], 'exist', 'targetRelation' => 'User'],
+            'user_id_exist' => [['user_id'], 'exist', 'targetRelation' => 'user'],
             'redelivery_of_integer' => [['redelivery_of'], 'integer'],
-            'redelivery_of_exist' => [['redelivery_of'], 'exist', 'targetRelation' => 'RedeliveryOf'],
+            'redelivery_of_exist' => [['redelivery_of'], 'exist', 'targetRelation' => 'redeliveryOf'],
             'name_string' => [['name'], 'string'],
         ];
     }
diff --git a/tests/specs/fk_col_name_index/app/models/base/Webhook.php b/tests/specs/fk_col_name_index/app/models/base/Webhook.php
index 29a40568..7adabbcc 100644
--- a/tests/specs/fk_col_name_index/app/models/base/Webhook.php
+++ b/tests/specs/fk_col_name_index/app/models/base/Webhook.php
@@ -27,11 +27,11 @@ public function rules()
         return [
             'trim' => [['name'], 'trim'],
             'user_id_integer' => [['user_id'], 'integer'],
-            'user_id_exist' => [['user_id'], 'exist', 'targetRelation' => 'User'],
+            'user_id_exist' => [['user_id'], 'exist', 'targetRelation' => 'user'],
             'redelivery_of_integer' => [['redelivery_of'], 'integer'],
-            'redelivery_of_exist' => [['redelivery_of'], 'exist', 'targetRelation' => 'RedeliveryOf'],
+            'redelivery_of_exist' => [['redelivery_of'], 'exist', 'targetRelation' => 'redeliveryOf'],
             'rd_abc_2_integer' => [['rd_abc_2'], 'integer'],
-            'rd_abc_2_exist' => [['rd_abc_2'], 'exist', 'targetRelation' => 'Rd2'],
+            'rd_abc_2_exist' => [['rd_abc_2'], 'exist', 'targetRelation' => 'rd2'],
             'user_id_name_unique' => [['user_id', 'name'], 'unique', 'targetAttribute' => [
                 'user_id',
                 'name',
diff --git a/tests/specs/issue_fix/159_bug_giiapi_generated_rules_emailid/maria/models/base/Contact.php b/tests/specs/issue_fix/159_bug_giiapi_generated_rules_emailid/maria/models/base/Contact.php
index efbd8b32..f77b3725 100644
--- a/tests/specs/issue_fix/159_bug_giiapi_generated_rules_emailid/maria/models/base/Contact.php
+++ b/tests/specs/issue_fix/159_bug_giiapi_generated_rules_emailid/maria/models/base/Contact.php
@@ -25,7 +25,7 @@ public function rules()
             'trim' => [['nickname'], 'trim'],
             'required' => [['mailing_id'], 'required'],
             'mailing_id_integer' => [['mailing_id'], 'integer'],
-            'mailing_id_exist' => [['mailing_id'], 'exist', 'targetRelation' => 'Mailing'],
+            'mailing_id_exist' => [['mailing_id'], 'exist', 'targetRelation' => 'mailing'],
             'active_boolean' => [['active'], 'boolean'],
             'active_default' => [['active'], 'default', 'value' => false],
             'nickname_string' => [['nickname'], 'string'],
diff --git a/tests/specs/issue_fix/162_bug_dollarref_with_x_faker/app/models/base/Order.php b/tests/specs/issue_fix/162_bug_dollarref_with_x_faker/app/models/base/Order.php
index 74472cd5..89d5b559 100644
--- a/tests/specs/issue_fix/162_bug_dollarref_with_x_faker/app/models/base/Order.php
+++ b/tests/specs/issue_fix/162_bug_dollarref_with_x_faker/app/models/base/Order.php
@@ -24,7 +24,7 @@ public function rules()
         return [
             'trim' => [['name', 'name2'], 'trim'],
             'invoice_id_integer' => [['invoice_id'], 'integer'],
-            'invoice_id_exist' => [['invoice_id'], 'exist', 'targetRelation' => 'Invoice'],
+            'invoice_id_exist' => [['invoice_id'], 'exist', 'targetRelation' => 'invoice'],
             'name_string' => [['name'], 'string'],
             'name2_string' => [['name2'], 'string'],
         ];
diff --git a/tests/specs/issue_fix/175_bug_allof_with_multiple_dollarrefs/pgsql/models/base/Contact.php b/tests/specs/issue_fix/175_bug_allof_with_multiple_dollarrefs/pgsql/models/base/Contact.php
index 21a2e7f1..ec235266 100644
--- a/tests/specs/issue_fix/175_bug_allof_with_multiple_dollarrefs/pgsql/models/base/Contact.php
+++ b/tests/specs/issue_fix/175_bug_allof_with_multiple_dollarrefs/pgsql/models/base/Contact.php
@@ -25,7 +25,7 @@ public function rules()
             'trim' => [['nickname'], 'trim'],
             'required' => [['account_id'], 'required'],
             'account_id_integer' => [['account_id'], 'integer'],
-            'account_id_exist' => [['account_id'], 'exist', 'targetRelation' => 'Account'],
+            'account_id_exist' => [['account_id'], 'exist', 'targetRelation' => 'account'],
             'active_boolean' => [['active'], 'boolean'],
             'active_default' => [['active'], 'default', 'value' => false],
             'nickname_string' => [['nickname'], 'string'],
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/index.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/index.php
new file mode 100644
index 00000000..f818842e
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/index.php
@@ -0,0 +1,13 @@
+<?php
+
+return [
+    'openApiPath' => '@specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/index.yaml',
+    'generateUrls' => false,
+    'generateModels' => true,
+    'excludeModels' => [
+        'Error',
+    ],
+    'generateControllers' => false,
+    'generateMigrations' => true,
+    'generateModelFaker' => true,
+];
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/index.yaml b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/index.yaml
new file mode 100644
index 00000000..6a872595
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/index.yaml
@@ -0,0 +1,33 @@
+openapi: "3.0.0"
+info:
+  version: 1.0.0
+  title: Extension FK COLUMN NAME cause error in case of column name without underscore \#29
+paths:
+  /:
+    get:
+      summary: List
+      operationId: list
+      responses:
+        '200':
+          description: The information
+
+components:
+  schemas:
+    User:
+      type: object
+      properties:
+        id:
+          type: integer
+        name:
+          type: string
+    Post:
+      type: object
+      properties:
+        id:
+          type: integer
+        content:
+          type: string
+        user:
+          allOf:
+            - $ref: '#/components/schemas/User'
+            - x-fk-column-name: user
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/migrations_mysql_db/m200000_000000_create_table_users.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/migrations_mysql_db/m200000_000000_create_table_users.php
new file mode 100644
index 00000000..9ab60f3e
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/migrations_mysql_db/m200000_000000_create_table_users.php
@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * Table for User
+ */
+class m200000_000000_create_table_users extends \yii\db\Migration
+{
+    public function up()
+    {
+        $this->createTable('{{%users}}', [
+            'id' => $this->primaryKey(),
+            'name' => $this->text()->null(),
+        ]);
+    }
+
+    public function down()
+    {
+        $this->dropTable('{{%users}}');
+    }
+}
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/migrations_mysql_db/m200000_000001_create_table_posts.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/migrations_mysql_db/m200000_000001_create_table_posts.php
new file mode 100644
index 00000000..ebb98b8e
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/migrations_mysql_db/m200000_000001_create_table_posts.php
@@ -0,0 +1,23 @@
+<?php
+
+/**
+ * Table for Post
+ */
+class m200000_000001_create_table_posts extends \yii\db\Migration
+{
+    public function up()
+    {
+        $this->createTable('{{%posts}}', [
+            'id' => $this->primaryKey(),
+            'content' => $this->text()->null(),
+            'user' => $this->integer()->null()->defaultValue(null),
+        ]);
+        $this->addForeignKey('fk_posts_user_users_id', '{{%posts}}', 'user', '{{%users}}', 'id');
+    }
+
+    public function down()
+    {
+        $this->dropForeignKey('fk_posts_user_users_id', '{{%posts}}');
+        $this->dropTable('{{%posts}}');
+    }
+}
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/BaseModelFaker.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/BaseModelFaker.php
new file mode 100644
index 00000000..c367fbb4
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/BaseModelFaker.php
@@ -0,0 +1,144 @@
+<?php
+
+namespace app\models;
+
+use Faker\Factory as FakerFactory;
+use Faker\Generator;
+use Faker\UniqueGenerator;
+
+/**
+ * Base fake data generator
+ */
+abstract class BaseModelFaker
+{
+    /**
+     * @var Generator
+    */
+    protected $faker;
+    /**
+     * @var UniqueGenerator
+    */
+    protected $uniqueFaker;
+
+    public function __construct()
+    {
+        $this->faker = FakerFactory::create(str_replace('-', '_', \Yii::$app->language));
+        $this->uniqueFaker = new UniqueGenerator($this->faker);
+    }
+
+    abstract public function generateModel($attributes = []);
+
+    public function getFaker():Generator
+    {
+        return $this->faker;
+    }
+
+    public function getUniqueFaker():UniqueGenerator
+    {
+        return $this->uniqueFaker;
+    }
+
+    public function setFaker(Generator $faker):void
+    {
+        $this->faker = $faker;
+    }
+
+    public function setUniqueFaker(UniqueGenerator $faker):void
+    {
+        $this->uniqueFaker = $faker;
+    }
+
+    /**
+     * Generate and return model
+     * @param array|callable $attributes
+     * @param UniqueGenerator|null $uniqueFaker
+     * @return \yii\db\ActiveRecord
+     * @example MyFaker::makeOne(['user_id' => 1, 'title' => 'foo']);
+     * @example MyFaker::makeOne( function($model, $faker) {
+     *        $model->scenario = 'create';
+     *        $model->setAttributes(['user_id' => 1, 'title' => $faker->sentence]);
+     *        return $model;
+     *  });
+     */
+    public static function makeOne($attributes = [], ?UniqueGenerator $uniqueFaker = null)
+    {
+        $fakeBuilder = new static();
+        if ($uniqueFaker !== null) {
+            $fakeBuilder->setUniqueFaker($uniqueFaker);
+        }
+        $model = $fakeBuilder->generateModel($attributes);
+        return $model;
+    }
+
+    /**
+     * Generate, save and return model
+     * @param array|callable $attributes
+     * @param UniqueGenerator|null $uniqueFaker
+     * @return \yii\db\ActiveRecord
+     * @example MyFaker::saveOne(['user_id' => 1, 'title' => 'foo']);
+     * @example MyFaker::saveOne( function($model, $faker) {
+     *        $model->scenario = 'create';
+     *        $model->setAttributes(['user_id' => 1, 'title' => $faker->sentence]);
+     *        return $model;
+     *  });
+     */
+    public static function saveOne($attributes = [], ?UniqueGenerator $uniqueFaker = null)
+    {
+        $model = static::makeOne($attributes, $uniqueFaker);
+        $model->save();
+        return $model;
+    }
+
+    /**
+     * Generate and return multiple models
+     * @param int $number
+     * @param array|callable $commonAttributes
+     * @return \yii\db\ActiveRecord[]|array
+     * @example TaskFaker::make(5, ['project_id'=>1, 'user_id' => 2]);
+     * @example TaskFaker::make(5, function($model, $faker, $uniqueFaker) {
+     *       $model->setAttributes(['name' => $uniqueFaker->username, 'state'=>$faker->boolean(20)]);
+     *       return $model;
+     * });
+     */
+    public static function make(int $number, $commonAttributes = [], ?UniqueGenerator $uniqueFaker = null):array
+    {
+        if ($number < 1) {
+            return [];
+        }
+        $fakeBuilder = new static();
+        if ($uniqueFaker !== null) {
+            $fakeBuilder->setUniqueFaker($uniqueFaker);
+        }
+        return array_map(function () use ($commonAttributes, $fakeBuilder) {
+            $model = $fakeBuilder->generateModel($commonAttributes);
+            return $model;
+        }, range(0, $number -1));
+    }
+
+    /**
+     * Generate, save and return multiple models
+     * @param int $number
+     * @param array|callable $commonAttributes
+     * @return \yii\db\ActiveRecord[]|array
+     * @example TaskFaker::save(5, ['project_id'=>1, 'user_id' => 2]);
+     * @example TaskFaker::save(5, function($model, $faker, $uniqueFaker) {
+     *       $model->setAttributes(['name' => $uniqueFaker->username, 'state'=>$faker->boolean(20)]);
+     *       return $model;
+     * });
+     */
+    public static function save(int $number, $commonAttributes = [], ?UniqueGenerator $uniqueFaker = null):array
+    {
+        if ($number < 1) {
+            return [];
+        }
+        $fakeBuilder = new static();
+        if ($uniqueFaker !== null) {
+            $fakeBuilder->setUniqueFaker($uniqueFaker);
+        }
+        return array_map(function () use ($commonAttributes, $fakeBuilder) {
+            $model = $fakeBuilder->generateModel($commonAttributes);
+            $model->save();
+            return $model;
+        }, range(0, $number -1));
+    }
+}
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/Post.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/Post.php
new file mode 100644
index 00000000..2825fe31
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/Post.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace app\models;
+
+class Post extends \app\models\base\Post
+{
+
+
+}
+
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/PostFaker.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/PostFaker.php
new file mode 100644
index 00000000..c595c8ca
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/PostFaker.php
@@ -0,0 +1,51 @@
+<?php
+namespace app\models;
+
+use Faker\UniqueGenerator;
+
+/**
+ * Fake data generator for Post
+ * @method static Post makeOne($attributes = [], ?UniqueGenerator $uniqueFaker = null);
+ * @method static Post saveOne($attributes = [], ?UniqueGenerator $uniqueFaker = null);
+ * @method static Post[] make(int $number, $commonAttributes = [], ?UniqueGenerator $uniqueFaker = null)
+ * @method static Post[] save(int $number, $commonAttributes = [], ?UniqueGenerator $uniqueFaker = null)
+ */
+class PostFaker extends BaseModelFaker
+{
+
+    /**
+     * @param array|callable $attributes
+     * @return Post|\yii\db\ActiveRecord
+     * @example
+     *  $model = (new PostFaker())->generateModels(['author_id' => 1]);
+     *  $model = (new PostFaker())->generateModels(function($model, $faker, $uniqueFaker) {
+     *            $model->scenario = 'create';
+     *            $model->author_id = 1;
+     *            return $model;
+     *  });
+    **/
+    public function generateModel($attributes = [])
+    {
+        $faker = $this->faker;
+        $uniqueFaker = $this->uniqueFaker;
+        $model = new Post();
+        //$model->id = $uniqueFaker->numberBetween(0, 1000000);
+        $model->content = $faker->paragraphs(6, true);
+        $model->user = $faker->randomElement(\app\models\User::find()->select("id")->column());
+        if (!is_callable($attributes)) {
+            $model->setAttributes($attributes, false);
+        } else {
+            $model = $attributes($model, $faker, $uniqueFaker);
+        }
+        return $model;
+    }
+
+    public static function dependentOn()
+    {
+        return [
+            // just model class names
+            'User',
+
+        ];
+    }
+}
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/User.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/User.php
new file mode 100644
index 00000000..9b837d6e
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/User.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace app\models;
+
+class User extends \app\models\base\User
+{
+
+
+}
+
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/UserFaker.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/UserFaker.php
new file mode 100644
index 00000000..5f389e83
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/UserFaker.php
@@ -0,0 +1,41 @@
+<?php
+namespace app\models;
+
+use Faker\UniqueGenerator;
+
+/**
+ * Fake data generator for User
+ * @method static User makeOne($attributes = [], ?UniqueGenerator $uniqueFaker = null);
+ * @method static User saveOne($attributes = [], ?UniqueGenerator $uniqueFaker = null);
+ * @method static User[] make(int $number, $commonAttributes = [], ?UniqueGenerator $uniqueFaker = null)
+ * @method static User[] save(int $number, $commonAttributes = [], ?UniqueGenerator $uniqueFaker = null)
+ */
+class UserFaker extends BaseModelFaker
+{
+
+    /**
+     * @param array|callable $attributes
+     * @return User|\yii\db\ActiveRecord
+     * @example
+     *  $model = (new PostFaker())->generateModels(['author_id' => 1]);
+     *  $model = (new PostFaker())->generateModels(function($model, $faker, $uniqueFaker) {
+     *            $model->scenario = 'create';
+     *            $model->author_id = 1;
+     *            return $model;
+     *  });
+    **/
+    public function generateModel($attributes = [])
+    {
+        $faker = $this->faker;
+        $uniqueFaker = $this->uniqueFaker;
+        $model = new User();
+        //$model->id = $uniqueFaker->numberBetween(0, 1000000);
+        $model->name = $faker->sentence;
+        if (!is_callable($attributes)) {
+            $model->setAttributes($attributes, false);
+        } else {
+            $model = $attributes($model, $faker, $uniqueFaker);
+        }
+        return $model;
+    }
+}
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/base/Post.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/base/Post.php
new file mode 100644
index 00000000..c8ba9f96
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/base/Post.php
@@ -0,0 +1,35 @@
+<?php
+
+namespace app\models\base;
+
+/**
+ *
+ *
+ * @property int $id
+ * @property string $content
+ * @property int $user
+ *
+ * @property \app\models\User $userRel
+ */
+abstract class Post extends \yii\db\ActiveRecord
+{
+    public static function tableName()
+    {
+        return '{{%posts}}';
+    }
+
+    public function rules()
+    {
+        return [
+            'trim' => [['content'], 'trim'],
+            'user_integer' => [['user'], 'integer'],
+            'user_exist' => [['user'], 'exist', 'targetRelation' => 'userRel'],
+            'content_string' => [['content'], 'string'],
+        ];
+    }
+
+    public function getUserRel()
+    {
+        return $this->hasOne(\app\models\User::class, ['id' => 'user']);
+    }
+}
diff --git a/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/base/User.php b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/base/User.php
new file mode 100644
index 00000000..08e59880
--- /dev/null
+++ b/tests/specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql/models/base/User.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace app\models\base;
+
+/**
+ *
+ *
+ * @property int $id
+ * @property string $name
+ *
+ */
+abstract class User extends \yii\db\ActiveRecord
+{
+    public static function tableName()
+    {
+        return '{{%users}}';
+    }
+
+    public function rules()
+    {
+        return [
+            'trim' => [['name'], 'trim'],
+            'name_string' => [['name'], 'string'],
+        ];
+    }
+}
diff --git a/tests/specs/issue_fix/model_name_more_than_once_in_faker_148/app/models/base/E123.php b/tests/specs/issue_fix/model_name_more_than_once_in_faker_148/app/models/base/E123.php
index cbec768a..ac4dce54 100644
--- a/tests/specs/issue_fix/model_name_more_than_once_in_faker_148/app/models/base/E123.php
+++ b/tests/specs/issue_fix/model_name_more_than_once_in_faker_148/app/models/base/E123.php
@@ -27,11 +27,11 @@ public function rules()
         return [
             'trim' => [['name'], 'trim'],
             'account_id_integer' => [['account_id'], 'integer'],
-            'account_id_exist' => [['account_id'], 'exist', 'targetRelation' => 'Account'],
+            'account_id_exist' => [['account_id'], 'exist', 'targetRelation' => 'account'],
             'account_2_id_integer' => [['account_2_id'], 'integer'],
-            'account_2_id_exist' => [['account_2_id'], 'exist', 'targetRelation' => 'Account2'],
+            'account_2_id_exist' => [['account_2_id'], 'exist', 'targetRelation' => 'account2'],
             'account_3_id_integer' => [['account_3_id'], 'integer'],
-            'account_3_id_exist' => [['account_3_id'], 'exist', 'targetRelation' => 'Account3'],
+            'account_3_id_exist' => [['account_3_id'], 'exist', 'targetRelation' => 'account3'],
             'name_string' => [['name'], 'string'],
         ];
     }
diff --git a/tests/specs/many2many/models/base/Photos2Posts.php b/tests/specs/many2many/models/base/Photos2Posts.php
index 523ecada..80ef4f97 100644
--- a/tests/specs/many2many/models/base/Photos2Posts.php
+++ b/tests/specs/many2many/models/base/Photos2Posts.php
@@ -23,9 +23,9 @@ public function rules()
     {
         return [
             'photo_id_integer' => [['photo_id'], 'integer'],
-            'photo_id_exist' => [['photo_id'], 'exist', 'targetRelation' => 'Photo'],
+            'photo_id_exist' => [['photo_id'], 'exist', 'targetRelation' => 'photo'],
             'post_id_integer' => [['post_id'], 'integer'],
-            'post_id_exist' => [['post_id'], 'exist', 'targetRelation' => 'Post'],
+            'post_id_exist' => [['post_id'], 'exist', 'targetRelation' => 'post'],
         ];
     }
 
diff --git a/tests/specs/many2many/models/base/PostsAttaches.php b/tests/specs/many2many/models/base/PostsAttaches.php
index 54b8adda..8b30ab15 100644
--- a/tests/specs/many2many/models/base/PostsAttaches.php
+++ b/tests/specs/many2many/models/base/PostsAttaches.php
@@ -23,9 +23,9 @@ public function rules()
     {
         return [
             'attach_id_integer' => [['attach_id'], 'integer'],
-            'attach_id_exist' => [['attach_id'], 'exist', 'targetRelation' => 'Attach'],
+            'attach_id_exist' => [['attach_id'], 'exist', 'targetRelation' => 'attach'],
             'target_id_integer' => [['target_id'], 'integer'],
-            'target_id_exist' => [['target_id'], 'exist', 'targetRelation' => 'Target'],
+            'target_id_exist' => [['target_id'], 'exist', 'targetRelation' => 'target'],
         ];
     }
 
diff --git a/tests/specs/many2many/models/base/PostsGallery.php b/tests/specs/many2many/models/base/PostsGallery.php
index 61cfd67f..0adea949 100644
--- a/tests/specs/many2many/models/base/PostsGallery.php
+++ b/tests/specs/many2many/models/base/PostsGallery.php
@@ -23,9 +23,9 @@ public function rules()
     {
         return [
             'image_id_integer' => [['image_id'], 'integer'],
-            'image_id_exist' => [['image_id'], 'exist', 'targetRelation' => 'Image'],
+            'image_id_exist' => [['image_id'], 'exist', 'targetRelation' => 'image'],
             'article_id_integer' => [['article_id'], 'integer'],
-            'article_id_exist' => [['article_id'], 'exist', 'targetRelation' => 'Article'],
+            'article_id_exist' => [['article_id'], 'exist', 'targetRelation' => 'article'],
             'is_cover_boolean' => [['is_cover'], 'boolean'],
         ];
     }
diff --git a/tests/specs/menu/models/base/Menu.php b/tests/specs/menu/models/base/Menu.php
index 582d952d..a92755f6 100644
--- a/tests/specs/menu/models/base/Menu.php
+++ b/tests/specs/menu/models/base/Menu.php
@@ -27,7 +27,7 @@ public function rules()
             'trim' => [['name'], 'trim'],
             'required' => [['name'], 'required'],
             'parent_id_integer' => [['parent_id'], 'integer'],
-            'parent_id_exist' => [['parent_id'], 'exist', 'targetRelation' => 'Parent'],
+            'parent_id_exist' => [['parent_id'], 'exist', 'targetRelation' => 'parent'],
             'name_string' => [['name'], 'string', 'min' => 3, 'max' => 100],
             'args_default' => [['args'], 'default', 'value' => [
                 'foo',
diff --git a/tests/specs/petstore/models/base/Pet.php b/tests/specs/petstore/models/base/Pet.php
index 039764d2..62f78428 100644
--- a/tests/specs/petstore/models/base/Pet.php
+++ b/tests/specs/petstore/models/base/Pet.php
@@ -25,7 +25,7 @@ public function rules()
             'trim' => [['name', 'tag'], 'trim'],
             'required' => [['name'], 'required'],
             'store_id_integer' => [['store_id'], 'integer'],
-            'store_id_exist' => [['store_id'], 'exist', 'targetRelation' => 'Store'],
+            'store_id_exist' => [['store_id'], 'exist', 'targetRelation' => 'store'],
             'name_string' => [['name'], 'string'],
             'tag_string' => [['tag'], 'string'],
         ];
diff --git a/tests/specs/petstore_jsonapi/models/base/PetStatistic.php b/tests/specs/petstore_jsonapi/models/base/PetStatistic.php
index ba8af41d..adc6e8fd 100644
--- a/tests/specs/petstore_jsonapi/models/base/PetStatistic.php
+++ b/tests/specs/petstore_jsonapi/models/base/PetStatistic.php
@@ -54,7 +54,7 @@ public function rules()
         return [
             'trim' => [['title', 'summary'], 'trim'],
             'parentPet_id_integer' => [['parentPet_id'], 'integer'],
-            'parentPet_id_exist' => [['parentPet_id'], 'exist', 'targetRelation' => 'ParentPet'],
+            'parentPet_id_exist' => [['parentPet_id'], 'exist', 'targetRelation' => 'parentPet'],
             'title_string' => [['title'], 'string'],
             'dogsCount_integer' => [['dogsCount'], 'integer'],
             'catsCount_integer' => [['catsCount'], 'integer'],
diff --git a/tests/specs/petstore_namespace/mymodels/base/Pet.php b/tests/specs/petstore_namespace/mymodels/base/Pet.php
index 58750645..cb549e8f 100644
--- a/tests/specs/petstore_namespace/mymodels/base/Pet.php
+++ b/tests/specs/petstore_namespace/mymodels/base/Pet.php
@@ -25,7 +25,7 @@ public function rules()
             'trim' => [['name', 'tag'], 'trim'],
             'required' => [['name'], 'required'],
             'store_id_integer' => [['store_id'], 'integer'],
-            'store_id_exist' => [['store_id'], 'exist', 'targetRelation' => 'Store'],
+            'store_id_exist' => [['store_id'], 'exist', 'targetRelation' => 'store'],
             'name_string' => [['name'], 'string'],
             'tag_string' => [['tag'], 'string'],
         ];
diff --git a/tests/specs/relations_in_faker/app/models/base/A123.php b/tests/specs/relations_in_faker/app/models/base/A123.php
index 9836dc3b..6aaf6df5 100644
--- a/tests/specs/relations_in_faker/app/models/base/A123.php
+++ b/tests/specs/relations_in_faker/app/models/base/A123.php
@@ -23,7 +23,7 @@ public function rules()
         return [
             'trim' => [['name'], 'trim'],
             'b123_id_integer' => [['b123_id'], 'integer'],
-            'b123_id_exist' => [['b123_id'], 'exist', 'targetRelation' => 'B123'],
+            'b123_id_exist' => [['b123_id'], 'exist', 'targetRelation' => 'b123'],
             'name_string' => [['name'], 'string'],
         ];
     }
diff --git a/tests/specs/relations_in_faker/app/models/base/B123.php b/tests/specs/relations_in_faker/app/models/base/B123.php
index f05fe320..8e5caa5a 100644
--- a/tests/specs/relations_in_faker/app/models/base/B123.php
+++ b/tests/specs/relations_in_faker/app/models/base/B123.php
@@ -23,7 +23,7 @@ public function rules()
         return [
             'trim' => [['name'], 'trim'],
             'c123_id_integer' => [['c123_id'], 'integer'],
-            'c123_id_exist' => [['c123_id'], 'exist', 'targetRelation' => 'C123'],
+            'c123_id_exist' => [['c123_id'], 'exist', 'targetRelation' => 'c123'],
             'name_string' => [['name'], 'string'],
         ];
     }
diff --git a/tests/specs/relations_in_faker/app/models/base/Domain.php b/tests/specs/relations_in_faker/app/models/base/Domain.php
index 3f861f9e..d378bbe3 100644
--- a/tests/specs/relations_in_faker/app/models/base/Domain.php
+++ b/tests/specs/relations_in_faker/app/models/base/Domain.php
@@ -26,7 +26,7 @@ public function rules()
             'trim' => [['name'], 'trim'],
             'required' => [['name', 'account_id'], 'required'],
             'account_id_integer' => [['account_id'], 'integer'],
-            'account_id_exist' => [['account_id'], 'exist', 'targetRelation' => 'Account'],
+            'account_id_exist' => [['account_id'], 'exist', 'targetRelation' => 'account'],
             'name_string' => [['name'], 'string', 'max' => 128],
         ];
     }
diff --git a/tests/specs/relations_in_faker/app/models/base/E123.php b/tests/specs/relations_in_faker/app/models/base/E123.php
index 6eb43814..c1a85c27 100644
--- a/tests/specs/relations_in_faker/app/models/base/E123.php
+++ b/tests/specs/relations_in_faker/app/models/base/E123.php
@@ -23,7 +23,7 @@ public function rules()
         return [
             'trim' => [['name'], 'trim'],
             'b123_id_integer' => [['b123_id'], 'integer'],
-            'b123_id_exist' => [['b123_id'], 'exist', 'targetRelation' => 'B123'],
+            'b123_id_exist' => [['b123_id'], 'exist', 'targetRelation' => 'b123'],
             'name_string' => [['name'], 'string'],
         ];
     }
diff --git a/tests/specs/relations_in_faker/app/models/base/Routing.php b/tests/specs/relations_in_faker/app/models/base/Routing.php
index 71a31b81..40e707e6 100644
--- a/tests/specs/relations_in_faker/app/models/base/Routing.php
+++ b/tests/specs/relations_in_faker/app/models/base/Routing.php
@@ -32,11 +32,11 @@ public function rules()
             'trim' => [['path', 'service'], 'trim'],
             'required' => [['domain_id'], 'required'],
             'domain_id_integer' => [['domain_id'], 'integer'],
-            'domain_id_exist' => [['domain_id'], 'exist', 'targetRelation' => 'Domain'],
+            'domain_id_exist' => [['domain_id'], 'exist', 'targetRelation' => 'domain'],
             'd123_id_integer' => [['d123_id'], 'integer'],
-            'd123_id_exist' => [['d123_id'], 'exist', 'targetRelation' => 'D123'],
+            'd123_id_exist' => [['d123_id'], 'exist', 'targetRelation' => 'd123'],
             'a123_id_integer' => [['a123_id'], 'integer'],
-            'a123_id_exist' => [['a123_id'], 'exist', 'targetRelation' => 'A123'],
+            'a123_id_exist' => [['a123_id'], 'exist', 'targetRelation' => 'a123'],
             'path_string' => [['path'], 'string', 'max' => 255],
             'ssl_boolean' => [['ssl'], 'boolean'],
             'redirect_to_ssl_boolean' => [['redirect_to_ssl'], 'boolean'],
diff --git a/tests/unit/IssueFixTest.php b/tests/unit/IssueFixTest.php
index b6c7abdb..ea9d2182 100644
--- a/tests/unit/IssueFixTest.php
+++ b/tests/unit/IssueFixTest.php
@@ -360,4 +360,18 @@ public function test158BugGiiapiGeneratedRulesEnumWithTrim()
         ]);
         $this->checkFiles($actualFiles, $expectedFiles);
     }
+
+    // https://github.com/php-openapi/yii2-openapi/issues/29
+    public function test29ExtensionFkColumnNameCauseErrorInCaseOfColumnNameWithoutUnderscore()
+    {
+        $testFile = Yii::getAlias("@specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/index.php");
+        $this->runGenerator($testFile);
+         $actualFiles = FileHelper::findFiles(Yii::getAlias('@app'), [
+             'recursive' => true,
+         ]);
+         $expectedFiles = FileHelper::findFiles(Yii::getAlias("@specs/issue_fix/29_extension_fk_column_name_cause_error_in_case_of_column_name_without_underscore/mysql"), [
+             'recursive' => true,
+         ]);
+         $this->checkFiles($actualFiles, $expectedFiles);
+    }
 }
diff --git a/tests/unit/ValidatorRulesBuilderTest.php b/tests/unit/ValidatorRulesBuilderTest.php
index 43343df6..af018565 100644
--- a/tests/unit/ValidatorRulesBuilderTest.php
+++ b/tests/unit/ValidatorRulesBuilderTest.php
@@ -48,7 +48,7 @@ public function testBuild()
             ], 'trim'),
             'required' => new ValidationRule(['title', 'category_id', 'required_with_def'], 'required'),
             'category_id_integer' => new ValidationRule(['category_id'], 'integer'),
-            'category_id_exist' => new ValidationRule(['category_id'], 'exist', ['targetRelation' => 'Category']),
+            'category_id_exist' => new ValidationRule(['category_id'], 'exist', ['targetRelation' => 'category']),
             'title_active_unique' => new ValidationRule(['title', 'active'], 'unique', [
                 'targetAttribute' =>
                     ['title', 'active'],