diff --git a/callback.go b/callback.go
index 6d0ff0f3..309078e4 100644
--- a/callback.go
+++ b/callback.go
@@ -148,75 +148,66 @@ func getRIndex(strs []string, str string) int {
 
 // sortProcessors sort callback processors based on its before, after, remove, replace
 func sortProcessors(cps []*CallbackProcessor) []*func(scope *Scope) {
-	var sortCallbackProcessor func(c *CallbackProcessor)
-	var names, sortedNames = []string{}, []string{}
+	var (
+		allNames, sortedNames []string
+		sortCallbackProcessor func(c *CallbackProcessor)
+	)
 
 	for _, cp := range cps {
-		if index := getRIndex(names, cp.name); index > -1 {
-			if !cp.replace && !cp.remove {
-				fmt.Printf("[warning] duplicated callback `%v` from %v\n", cp.name, fileWithLineNum())
-			}
+		// show warning message the callback name already exists
+		if index := getRIndex(allNames, cp.name); index > -1 && !cp.replace && !cp.remove {
+			fmt.Printf("[warning] duplicated callback `%v` from %v\n", cp.name, fileWithLineNum())
 		}
-		names = append(names, cp.name)
+		allNames = append(allNames, cp.name)
 	}
 
 	sortCallbackProcessor = func(c *CallbackProcessor) {
-		if getRIndex(sortedNames, c.name) > -1 {
-			return
-		}
-
-		if len(c.before) > 0 {
-			if index := getRIndex(sortedNames, c.before); index > -1 {
-				sortedNames = append(sortedNames[:index], append([]string{c.name}, sortedNames[index:]...)...)
-			} else if index := getRIndex(names, c.before); index > -1 {
-				sortedNames = append(sortedNames, c.name)
-				sortCallbackProcessor(cps[index])
-			} else {
-				sortedNames = append(sortedNames, c.name)
-			}
-		}
-
-		if len(c.after) > 0 {
-			if index := getRIndex(sortedNames, c.after); index > -1 {
-				sortedNames = append(sortedNames[:index+1], append([]string{c.name}, sortedNames[index+1:]...)...)
-			} else if index := getRIndex(names, c.after); index > -1 {
-				cp := cps[index]
-				if len(cp.before) == 0 {
-					cp.before = c.name
+		if getRIndex(sortedNames, c.name) == -1 { // if not sorted
+			if c.before != "" { // if defined before callback
+				if index := getRIndex(sortedNames, c.before); index != -1 {
+					// if before callback already sorted, append current callback just after it
+					sortedNames = append(sortedNames[:index], append([]string{c.name}, sortedNames[index:]...)...)
+				} else if index := getRIndex(allNames, c.before); index != -1 {
+					// if before callback exists but haven't sorted, append current callback to last
+					sortedNames = append(sortedNames, c.name)
+					sortCallbackProcessor(cps[index])
 				}
-				sortCallbackProcessor(cp)
-			} else {
+			}
+
+			if c.after != "" { // if defined after callback
+				if index := getRIndex(sortedNames, c.after); index != -1 {
+					// if after callback already sorted, append current callback just before it
+					sortedNames = append(sortedNames[:index+1], append([]string{c.name}, sortedNames[index+1:]...)...)
+				} else if index := getRIndex(allNames, c.after); index != -1 {
+					// if after callback exists but haven't sorted
+					cp := cps[index]
+					// set after callback's before callback to current callback
+					if cp.before == "" {
+						cp.before = c.name
+					}
+					sortCallbackProcessor(cp)
+				}
+			}
+
+			// if current callback haven't been sorted, append it to last
+			if getRIndex(sortedNames, c.name) == -1 {
 				sortedNames = append(sortedNames, c.name)
 			}
 		}
-
-		if getRIndex(sortedNames, c.name) == -1 {
-			sortedNames = append(sortedNames, c.name)
-		}
 	}
 
 	for _, cp := range cps {
 		sortCallbackProcessor(cp)
 	}
 
-	var funcs = []*func(scope *Scope){}
-	var sortedFuncs = []*func(scope *Scope){}
+	var sortedFuncs []*func(scope *Scope)
 	for _, name := range sortedNames {
-		index := getRIndex(names, name)
-		if !cps[index].remove {
+		if index := getRIndex(allNames, name); !cps[index].remove {
 			sortedFuncs = append(sortedFuncs, cps[index].processor)
 		}
 	}
 
-	for _, cp := range cps {
-		if sindex := getRIndex(sortedNames, cp.name); sindex == -1 {
-			if !cp.remove {
-				funcs = append(funcs, cp.processor)
-			}
-		}
-	}
-
-	return append(sortedFuncs, funcs...)
+	return sortedFuncs
 }
 
 // reorder all registered processors, and reset CURD callbacks
diff --git a/callback_delete.go b/callback_delete.go
index 17b5cfb4..b3a77926 100644
--- a/callback_delete.go
+++ b/callback_delete.go
@@ -2,6 +2,7 @@ package gorm
 
 import "fmt"
 
+// Define callbacks for deleting
 func init() {
 	defaultCallback.Delete().Register("gorm:begin_transaction", beginTransactionCallback)
 	defaultCallback.Delete().Register("gorm:before_delete", beforeDeleteCallback)
@@ -10,12 +11,14 @@ func init() {
 	defaultCallback.Delete().Register("gorm:commit_or_rollback_transaction", commitOrRollbackTransactionCallback)
 }
 
+// beforeDeleteCallback will invoke `BeforeDelete` method before deleting
 func beforeDeleteCallback(scope *Scope) {
 	if !scope.HasError() {
 		scope.CallMethod("BeforeDelete")
 	}
 }
 
+// deleteCallback used to delete data from database or set deleted_at to current time (when using with soft delete)
 func deleteCallback(scope *Scope) {
 	if !scope.HasError() {
 		if !scope.Search.Unscoped && scope.HasColumn("DeletedAt") {
@@ -24,15 +27,14 @@ func deleteCallback(scope *Scope) {
 					scope.QuotedTableName(),
 					scope.AddToVars(NowFunc()),
 					scope.CombinedConditionSql(),
-				))
+				)).Exec()
 		} else {
-			scope.Raw(fmt.Sprintf("DELETE FROM %v %v", scope.QuotedTableName(), scope.CombinedConditionSql()))
+			scope.Raw(fmt.Sprintf("DELETE FROM %v %v", scope.QuotedTableName(), scope.CombinedConditionSql())).Exec()
 		}
-
-		scope.Exec()
 	}
 }
 
+// afterDeleteCallback will invoke `AfterDelete` method after deleting
 func afterDeleteCallback(scope *Scope) {
 	if !scope.HasError() {
 		scope.CallMethod("AfterDelete")
diff --git a/callback_query.go b/callback_query.go
index f6fa0aa1..5d9fd82d 100644
--- a/callback_query.go
+++ b/callback_query.go
@@ -6,40 +6,42 @@ import (
 	"reflect"
 )
 
+// Define callbacks for querying
 func init() {
 	defaultCallback.Query().Register("gorm:query", queryCallback)
 	defaultCallback.Query().Register("gorm:after_query", afterQueryCallback)
 	defaultCallback.Query().Register("gorm:preload", preloadCallback)
 }
 
+// queryCallback used to query data from database
 func queryCallback(scope *Scope) {
 	defer scope.trace(NowFunc())
 
 	var (
-		isSlice  bool
-		isPtr    bool
-		destType reflect.Type
+		isSlice    bool
+		isPtr      bool
+		results    = scope.IndirectValue()
+		resultType reflect.Type
 	)
 
 	if orderBy, ok := scope.Get("gorm:order_by_primary_key"); ok {
-		if primaryKey := scope.PrimaryKey(); primaryKey != "" {
-			scope.Search.Order(fmt.Sprintf("%v.%v %v", scope.QuotedTableName(), scope.Quote(primaryKey), orderBy))
+		if primaryField := scope.PrimaryField(); primaryField != nil {
+			scope.Search.Order(fmt.Sprintf("%v.%v %v", scope.QuotedTableName(), scope.Quote(primaryField.DBName), orderBy))
 		}
 	}
 
-	var dest = scope.IndirectValue()
 	if value, ok := scope.Get("gorm:query_destination"); ok {
-		dest = reflect.Indirect(reflect.ValueOf(value))
+		results = reflect.Indirect(reflect.ValueOf(value))
 	}
 
-	if kind := dest.Kind(); kind == reflect.Slice {
+	if kind := results.Kind(); kind == reflect.Slice {
 		isSlice = true
-		destType = dest.Type().Elem()
-		dest.Set(reflect.MakeSlice(dest.Type(), 0, 0))
+		resultType = results.Type().Elem()
+		results.Set(reflect.MakeSlice(results.Type(), 0, 0))
 
-		if destType.Kind() == reflect.Ptr {
+		if resultType.Kind() == reflect.Ptr {
 			isPtr = true
-			destType = destType.Elem()
+			resultType = resultType.Elem()
 		}
 	} else if kind != reflect.Struct {
 		scope.Err(errors.New("unsupported destination, should be slice or struct"))
@@ -49,41 +51,38 @@ func queryCallback(scope *Scope) {
 	scope.prepareQuerySql()
 
 	if !scope.HasError() {
-		rows, err := scope.SqlDB().Query(scope.Sql, scope.SqlVars...)
 		scope.db.RowsAffected = 0
+		if rows, err := scope.SqlDB().Query(scope.Sql, scope.SqlVars...); scope.Err(err) == nil {
+			defer rows.Close()
 
-		if scope.Err(err) != nil {
-			return
-		}
-		defer rows.Close()
+			columns, _ := rows.Columns()
+			for rows.Next() {
+				scope.db.RowsAffected++
 
-		columns, _ := rows.Columns()
-		for rows.Next() {
-			scope.db.RowsAffected++
+				elem := results
+				if isSlice {
+					elem = reflect.New(resultType).Elem()
+				}
 
-			elem := dest
-			if isSlice {
-				elem = reflect.New(destType).Elem()
-			}
+				scope.scan(rows, columns, scope.New(elem.Addr().Interface()).Fields())
 
-			fields := scope.New(elem.Addr().Interface()).Fields()
-			scope.scan(rows, columns, fields)
-
-			if isSlice {
-				if isPtr {
-					dest.Set(reflect.Append(dest, elem.Addr()))
-				} else {
-					dest.Set(reflect.Append(dest, elem))
+				if isSlice {
+					if isPtr {
+						results.Set(reflect.Append(results, elem.Addr()))
+					} else {
+						results.Set(reflect.Append(results, elem))
+					}
 				}
 			}
-		}
 
-		if scope.db.RowsAffected == 0 && !isSlice {
-			scope.Err(RecordNotFound)
+			if scope.db.RowsAffected == 0 && !isSlice {
+				scope.Err(RecordNotFound)
+			}
 		}
 	}
 }
 
+// afterQueryCallback will invoke `AfterFind` method after querying
 func afterQueryCallback(scope *Scope) {
 	if !scope.HasError() {
 		scope.CallMethod("AfterFind")
diff --git a/callback_query_preload.go b/callback_query_preload.go
index 5dc91de9..ff99fea9 100644
--- a/callback_query_preload.go
+++ b/callback_query_preload.go
@@ -7,6 +7,7 @@ import (
 	"strings"
 )
 
+// preloadCallback used to preload associations
 func preloadCallback(scope *Scope) {
 	if scope.Search.preload == nil || scope.HasError() {
 		return
@@ -72,6 +73,7 @@ func preloadCallback(scope *Scope) {
 	}
 }
 
+// handleHasOnePreload used to preload has one associations
 func (scope *Scope) handleHasOnePreload(field *Field, conditions []interface{}) {
 	relation := field.Relationship
 
@@ -107,6 +109,7 @@ func (scope *Scope) handleHasOnePreload(field *Field, conditions []interface{})
 	}
 }
 
+// handleHasManyPreload used to preload has many associations
 func (scope *Scope) handleHasManyPreload(field *Field, conditions []interface{}) {
 	relation := field.Relationship
 
@@ -144,6 +147,7 @@ func (scope *Scope) handleHasManyPreload(field *Field, conditions []interface{})
 	}
 }
 
+// handleBelongsToPreload used to preload belongs to associations
 func (scope *Scope) handleBelongsToPreload(field *Field, conditions []interface{}) {
 	relation := field.Relationship
 
@@ -179,6 +183,7 @@ func (scope *Scope) handleBelongsToPreload(field *Field, conditions []interface{
 	}
 }
 
+// handleManyToManyPreload used to preload many to many associations
 func (scope *Scope) handleManyToManyPreload(field *Field, conditions []interface{}) {
 	var (
 		relation         = field.Relationship