Call Query callback chain when preloading many2many

When using `Preload` on a `many2many` association, the
`Query` callback chain was not being called. This made
it difficult to write a plugin that could reliably get
called regardless of how objects were being queried.

Now `handleManyToManyPreload` will call the `Query`
callback chain for each object that is retrieved by
following the association.

Since the data has already been read by the
`handleManyToManyPreload` method, a new scope setting
called `gorm:skip_queryCallback` is set to `true` before
calling the callbacks. Callbacks can check for the
presence of this setting if they should not be run; the
default `queryCallback` is an example of this case.

Fixes jinzhu/gorm#1621.

callback_query.go

@@ -15,6 +15,10 @@ func init() {
 
 // queryCallback used to query data from database
 func queryCallback(scope *Scope) {
+	if _, skip := scope.Get("gorm:skip_query_callback"); skip {
+		return
+	}
+
 	defer scope.trace(NowFunc())
 
 	var (

callback_query_preload.go

@@ -324,6 +324,10 @@ func (scope *Scope) handleManyToManyPreload(field *Field, conditions []interface
 
 		scope.scan(rows, columns, append(fields, joinTableFields...))
 
+		scope.New(elem.Addr().Interface()).
+			Set("gorm:skip_query_callback", true).
+			callCallbacks(scope.db.parent.callbacks.queries)
+
 		var foreignKeys = make([]interface{}, len(sourceKeys))
 		// generate hashed forkey keys in join table
 		for idx, joinTableField := range joinTableFields {

preload_test.go

@@ -1627,6 +1627,46 @@ func TestPrefixedPreloadDuplication(t *testing.T) {
 	}
 }
 
+func TestPreloadManyToManyCallbacks(t *testing.T) {
+	type (
+		Level2 struct {
+			ID uint
+		}
+		Level1 struct {
+			ID      uint
+			Level2s []Level2 `gorm:"many2many:level1_level2s;AssociationForeignKey:ID;ForeignKey:ID"`
+		}
+	)
+
+	DB.DropTableIfExists("level1_level2s")
+	DB.DropTableIfExists(new(Level1))
+	DB.DropTableIfExists(new(Level2))
+
+	if err := DB.AutoMigrate(new(Level1), new(Level2)).Error; err != nil {
+		t.Error(err)
+	}
+
+	lvl := Level1{
+		Level2s: []Level2{
+			Level2{},
+		},
+	}
+	DB.Save(&lvl)
+
+	called := 0
+
+	DB.Callback().Query().After("gorm:query").Register("TestPreloadManyToManyCallbacks", func(scope *gorm.Scope) {
+		called = called + 1
+	})
+
+	found := Level1{ID: lvl.ID}
+	DB.Preload("Level2s").First(&found, &found)
+
+	if called != 2 {
+		t.Errorf("Wanted callback to be called 2 times but got %d", called)
+	}
+}
+
 func toJSONString(v interface{}) []byte {
 	r, _ := json.MarshalIndent(v, "", "  ")
 	return r