[5.4] Fix BelongsTo not accepting id == 0 in foreign relations

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

@@ -116,12 +116,16 @@ public function addEagerConstraints(array $models)
      */
     protected function getEagerModelKeys(array $models)
     {
+        $keys = [];
+
         // First we need to gather all of the keys from the parent models so we know what
         // to query for via the eager loading query. We will add them to an array then
         // execute a "where in" statement to gather up all of those related records.
-        $keys = collect($models)->map(function ($model) {
-            return $model->{$this->foreignKey};
-        })->filter()->all();
+        foreach ($models as $model) {
+            if (! is_null($value = $model->{$this->foreignKey})) {
+                $keys[] = $value;
+            }
+        }
 
         // If there are no keys that were not null we will just return an array with either
         // null or 0 in (depending on if incrementing keys are in use) so the query wont

tests/Database/DatabaseEloquentBelongsToTest.php

@@ -33,6 +33,14 @@ public function testEagerConstraintsAreProperlyAdded()
         $relation->addEagerConstraints($models);
     }
 
+    public function testIdsInEagerConstraintsCanBeZero()
+    {
+        $relation = $this->getRelation();
+        $relation->getQuery()->shouldReceive('whereIn')->once()->with('relation.id', ['foreign.value', 0]);
+        $models = [new EloquentBelongsToModelStub, new EloquentBelongsToModelStubWithZeroId];
+        $relation->addEagerConstraints($models);
+    }
+
     public function testRelationIsProperlyInitialized()
     {
         $relation = $this->getRelation();
@@ -143,6 +151,11 @@ class AnotherEloquentBelongsToModelStub extends \Illuminate\Database\Eloquent\Mo
     public $foreign_key = 'foreign.value.two';
 }
 
+class EloquentBelongsToModelStubWithZeroId extends \Illuminate\Database\Eloquent\Model
+{
+    public $foreign_key = 0;
+}
+
 class MissingEloquentBelongsToModelStub extends \Illuminate\Database\Eloquent\Model
 {
     public $foreign_key;