diff --git a/src/HasManyMerged.php b/src/HasManyMerged.php
index 2a27676..fb582bc 100644
--- a/src/HasManyMerged.php
+++ b/src/HasManyMerged.php
@@ -73,6 +73,16 @@ public function getParentKey()
         return $this->parent->getAttribute($this->localKey);
     }
 
+    /**
+     * Get the fully qualified parent key name.
+     *
+     * @return string
+     */
+    public function getQualifiedParentKeyName()
+    {
+        return $this->parent->qualifyColumn($this->localKey);
+    }
+
     /**
      * Set the constraints for an eager load of the relation.
      * Note: Used to load relations of multiple models at once.
@@ -91,6 +101,30 @@ public function addEagerConstraints(array $models)
         });
     }
 
+    /**
+     * Add the constraints for an internal relationship existence query.
+     *
+     * Essentially, these queries compare on column names like whereColumn.
+     *
+     * @param  \Illuminate\Database\Eloquent\Builder  $query
+     * @param  \Illuminate\Database\Eloquent\Builder  $parentQuery
+     * @param  array|mixed  $columns
+     * @return \Illuminate\Database\Eloquent\Builder
+     */
+    public function getRelationExistenceQuery(Builder $query, Builder $parentQuery, $columns = ['*'])
+    {
+        $foreignKeys = $this->foreignKeys;
+
+        return $query->select($columns)->where(function ($query) use ($foreignKeys) {
+            foreach ($foreignKeys as $foreignKey) {
+                $query->orWhere(function ($query) use ($foreignKey) {
+                    $query->whereColumn($this->getQualifiedParentKeyName(), '=', $foreignKey)
+                        ->whereNotNull($foreignKey);
+                });
+            }
+        });
+    }
+
     /**
      * Get the name of the "where in" method for eager loading.
      * Note: Similar to whereInMethod of Relation class.
diff --git a/tests/HasManyMergedTest.php b/tests/HasManyMergedTest.php
index 8a2cb23..7cdb5ae 100644
--- a/tests/HasManyMergedTest.php
+++ b/tests/HasManyMergedTest.php
@@ -2,13 +2,14 @@
 
 namespace Korridor\LaravelHasManyMerged\Tests;
 
+use Illuminate\Database\Eloquent\Builder;
 use Korridor\LaravelHasManyMerged\HasManyMerged;
 use Korridor\LaravelHasManyMerged\Tests\Models\Message;
 use Korridor\LaravelHasManyMerged\Tests\Models\User;
 
 class HasManyMergedTest extends TestCase
 {
-    private function createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages()
+    private function createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages(): void
     {
         User::create([
             'id' => 11,
@@ -26,28 +27,32 @@ private function createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages(
             'content' => 'A - This is a message!',
             'sender_user_id' => 1,
             'receiver_user_id' => 1,
+            'content_integer' => 1,
         ]);
         Message::create([
             'id' => 2,
             'content' => 'B - This is a message!',
             'sender_user_id' => 1,
             'receiver_user_id' => 2,
+            'content_integer' => 1,
         ]);
         Message::create([
             'id' => 3,
             'content' => 'C - This is a message!',
             'sender_user_id' => 2,
             'receiver_user_id' => 1,
+            'content_integer' => 1,
         ]);
         Message::create([
             'id' => 4,
             'content' => 'D - This is a message!',
             'sender_user_id' => 2,
             'receiver_user_id' => 2,
+            'content_integer' => 2,
         ]);
     }
 
-    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithLazyLoading()
+    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithLazyLoading(): void
     {
         // Arrange
         $this->createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages();
@@ -71,7 +76,7 @@ public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSam
         $this->assertEquals(2, $user2->sentMessages()->count());
     }
 
-    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithEagerLoading()
+    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithEagerLoading(): void
     {
         // Arrange
         $this->createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages();
@@ -96,7 +101,83 @@ public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSam
         $this->assertEquals(2, $user2->sentMessages()->count());
     }
 
-    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithLazyEagerLoading()
+    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithEagerLoadingWithCount(): void
+    {
+        // Arrange
+        $this->createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages();
+
+        // Act
+        $this->db::connection()->enableQueryLog();
+        $users = User::with(['messages'])->withCount(['messages', 'sentMessages'])->get();
+        $user1 = $users->firstWhere('id', 11);
+        $user2 = $users->firstWhere('id', 12);
+        $messagesOfUser1 = $user1->messages;
+        $messagesOfUser2 = $user2->messages;
+        $queries = $this->db::getQueryLog();
+        $this->db::connection()->disableQueryLog();
+
+        // Assert
+        $this->assertEquals(2, count($queries));
+        $this->assertEquals(3, $messagesOfUser1->count());
+        $this->assertEquals(3, $messagesOfUser2->count());
+        $this->assertEquals(3, $user1->messages_count);
+        $this->assertEquals(3, $user2->messages_count);
+        $this->assertEquals(2, $user1->receivedMessages()->count());
+        $this->assertEquals(2, $user1->sentMessages()->count());
+        $this->assertEquals(2, $user2->receivedMessages()->count());
+        $this->assertEquals(2, $user2->sentMessages()->count());
+    }
+
+    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithEagerLoadingWithSum(): void
+    {
+        if (! method_exists(Builder::class, 'withSum')) {
+            $this->markTestSkipped('withSum is not supported');
+        }
+
+        // Arrange
+        $this->createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages();
+
+        // Act
+        $this->db::connection()->enableQueryLog();
+        $users = User::with(['messages'])->withSum('messages', 'content_integer')->get();
+        $user1 = $users->firstWhere('id', 11);
+        $user2 = $users->firstWhere('id', 12);
+        $messagesOfUser1 = $user1->messages;
+        $messagesOfUser2 = $user2->messages;
+        $queries = $this->db::getQueryLog();
+        $this->db::connection()->disableQueryLog();
+
+        // Assert
+        $this->assertEquals(2, count($queries));
+        $this->assertEquals(3, $messagesOfUser1->count());
+        $this->assertEquals(3, $messagesOfUser2->count());
+        $this->assertEquals(3, $user1->messages_sum_content_integer);
+        $this->assertEquals(4, $user2->messages_sum_content_integer);
+        $this->assertEquals(2, $user1->receivedMessages()->count());
+        $this->assertEquals(2, $user1->sentMessages()->count());
+        $this->assertEquals(2, $user2->receivedMessages()->count());
+        $this->assertEquals(2, $user2->sentMessages()->count());
+    }
+
+    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithEagerLoadingWhereHas(): void
+    {
+        // Arrange
+        $this->createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages();
+
+        // Act
+        $this->db::connection()->enableQueryLog();
+        $users = User::with(['messages'])->whereHas('messages', function (Builder $builder) {
+            $builder->where('content_integer', '=', 2);
+        })->get();
+        $queries = $this->db::getQueryLog();
+        $this->db::connection()->disableQueryLog();
+
+        // Assert
+        $this->assertEquals(1, count($users));
+        $this->assertEquals(2, $users->first()->other_unique_id);
+    }
+
+    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithLazyEagerLoading(): void
     {
         // Arrange
         $this->createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages();
@@ -122,7 +203,7 @@ public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSam
         $this->assertEquals(2, $user2->sentMessages()->count());
     }
 
-    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithConstrainedEagerLoading()
+    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithConstrainedEagerLoading(): void
     {
         // Arrange
         $this->createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages();
@@ -151,7 +232,7 @@ public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSam
         $this->assertEquals(2, $user2->sentMessages()->count());
     }
 
-    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithEagerLoadingAndOrderCheck()
+    public function testHasManyMergedWithTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessagesWithEagerLoadingAndOrderCheck(): void
     {
         // Arrange
         $this->createTwoUsersWereBothAreSenderOrReceiverOfTheSameFourMessages();
diff --git a/tests/Models/Message.php b/tests/Models/Message.php
index 6ddbddb..3327d75 100644
--- a/tests/Models/Message.php
+++ b/tests/Models/Message.php
@@ -18,6 +18,7 @@ class Message extends Model
     protected $fillable = [
         'id',
         'content',
+        'content_integer',
         'sender_user_id',
         'receiver_user_id',
     ];
diff --git a/tests/TestCase.php b/tests/TestCase.php
index b7077d9..104302a 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -34,6 +34,7 @@ protected function setUp(): void
         $this->db::schema()->create('messages', function (Blueprint $table) {
             $table->increments('id');
             $table->text('content');
+            $table->integer('content_integer')->default(0);
             $table->unsignedInteger('sender_user_id');
             $table->unsignedInteger('receiver_user_id');
             $table->timestamps();