[10.x] Optimize eager loading when no keys to be loaded (#43724)

* Optimize eager loading when no keys to be loaded

* Add eagerImplicitlyEmpty parameter and docblock comment.

* Remove unneeded default in if statement for eagerImplicitlyEmpty

* formatting

Co-authored-by: Taylor Otwell <[email protected]>

Author: garygreen

src/Illuminate/Database/Eloquent/Relations/BelongsTo.php

@@ -113,7 +113,7 @@ public function addEagerConstraints(array $models)
 
         $whereIn = $this->whereInMethod($this->related, $this->ownerKey);
 
-        $this->query->{$whereIn}($key, $this->getEagerModelKeys($models));
+        $this->whereInEager($whereIn, $key, $this->getEagerModelKeys($models));
     }
 
     /**

src/Illuminate/Database/Eloquent/Relations/BelongsToMany.php

@@ -242,7 +242,8 @@ public function addEagerConstraints(array $models)
     {
         $whereIn = $this->whereInMethod($this->parent, $this->parentKey);
 
-        $this->query->{$whereIn}(
+        $this->whereInEager(
+            $whereIn,
             $this->getQualifiedForeignPivotKeyName(),
             $this->getKeys($models, $this->parentKey)
         );

src/Illuminate/Database/Eloquent/Relations/HasManyThrough.php

@@ -160,8 +160,10 @@ public function addEagerConstraints(array $models)
     {
         $whereIn = $this->whereInMethod($this->farParent, $this->localKey);
 
-        $this->query->{$whereIn}(
-            $this->getQualifiedFirstKeyName(), $this->getKeys($models, $this->localKey)
+        $this->whereInEager(
+            $whereIn,
+            $this->getQualifiedFirstKeyName(),
+            $this->getKeys($models, $this->localKey)
         );
     }
 

src/Illuminate/Database/Eloquent/Relations/HasOneOrMany.php

@@ -98,8 +98,11 @@ public function addEagerConstraints(array $models)
     {
         $whereIn = $this->whereInMethod($this->parent, $this->localKey);
 
-        $this->getRelationQuery()->{$whereIn}(
-            $this->foreignKey, $this->getKeys($models, $this->localKey)
+        $this->whereInEager(
+            $whereIn,
+            $this->foreignKey,
+            $this->getKeys($models, $this->localKey),
+            $this->getRelationQuery()
         );
     }
 

src/Illuminate/Database/Eloquent/Relations/Relation.php

@@ -40,6 +40,13 @@ abstract class Relation implements BuilderContract
      */
     protected $related;
 
+    /**
+     * Indicates whether the eagerly loaded relation should implicitly return an empty collection.
+     *
+     * @var bool
+     */
+    protected $eagerKeysWereEmpty = false;
+
     /**
      * Indicates if the relation is adding constraints.
      *
@@ -154,7 +161,9 @@ abstract public function getResults();
      */
     public function getEager()
     {
-        return $this->get();
+        return $this->eagerKeysWereEmpty
+                    ? $this->query->getModel()->newCollection()
+                    : $this->get();
     }
 
     /**
@@ -377,6 +386,24 @@ public function relatedUpdatedAt()
         return $this->related->getUpdatedAtColumn();
     }
 
+    /**
+     * Add a whereIn eager constraint for the given set of model keys to be loaded.
+     *
+     * @param  string  $whereIn
+     * @param  string  $key
+     * @param  array  $modelKeys
+     * @param  \Illuminate\Database\Eloquent\Builder  $query
+     * @return void
+     */
+    protected function whereInEager(string $whereIn, string $key, array $modelKeys, $query = null)
+    {
+        ($query ?? $this->query)->{$whereIn}($key, $modelKeys);
+
+        if ($modelKeys === []) {
+            $this->eagerKeysWereEmpty = true;
+        }
+    }
+
     /**
      * Get the name of the "where in" method for eager loading.
      *

tests/Database/DatabaseEloquentBuilderTest.php

@@ -767,6 +767,29 @@ public function testRelationshipEagerLoadProcess()
         unset($_SERVER['__eloquent.constrain']);
     }
 
+    public function testRelationshipEagerLoadProcessForImplicitlyEmpty()
+    {
+        $queryBuilder = $this->getMockQueryBuilder();
+        $builder = m::mock(Builder::class.'[getRelation]', [$queryBuilder]);
+        $builder->setEagerLoads(['parentFoo' => function ($query) {
+            $_SERVER['__eloquent.constrain'] = $query;
+        }]);
+        $model = new EloquentBuilderTestModelSelfRelatedStub;
+        $this->mockConnectionForModel($model, 'SQLite');
+
+        $models = [
+            new EloquentBuilderTestModelSelfRelatedStub,
+            new EloquentBuilderTestModelSelfRelatedStub,
+        ];
+        $relation = m::mock($model->parentFoo());
+
+        $builder->shouldReceive('getRelation')->once()->with('parentFoo')->andReturn($relation);
+
+        $results = $builder->eagerLoadRelations($models);
+
+        unset($_SERVER['__eloquent.constrain']);
+    }
+
     public function testGetRelationProperlySetsNestedRelationships()
     {
         $builder = $this->getBuilder();