Proposal, Add Specific on for Joins queries

callbacks/query.go

@@ -125,6 +125,7 @@ func BuildQuerySQL(db *gorm.DB) {
 						})
 					}
 
+					if join.On != nil {
 						exprs := make([]clause.Expression, len(relation.References))
 						for idx, ref := range relation.References {
 							if ref.OwnPrimaryKey {
@@ -146,12 +147,18 @@ func BuildQuerySQL(db *gorm.DB) {
 								}
 							}
 						}
-
 						joins = append(joins, clause.Join{
 							Type:  clause.LeftJoin,
 							Table: clause.Table{Name: relation.FieldSchema.Table, Alias: tableAliasName},
 							ON:    clause.Where{Exprs: exprs},
 						})
+					} else {
+						joins = append(joins, clause.Join{
+							Type:  clause.LeftJoin,
+							Table: clause.Table{Name: relation.FieldSchema.Table, Alias: tableAliasName},
+							ON:    clause.Where{Exprs: []clause.Expression{join.On}},
+						})
+					}
 				} else {
 					joins = append(joins, clause.Join{
 						Expression: clause.NamedExpr{SQL: join.Name, Vars: join.Conds},

chainable_api.go

@@ -177,6 +177,12 @@ func (db *DB) Joins(query string, args ...interface{}) (tx *DB) {
 	return
 }
 
+func (db *DB) JoinsOn(query string, on clause.Expression, args ...interface{}) (tx *DB) {
+	tx = db.getInstance()
+	tx.Statement.Joins = append(tx.Statement.Joins, join{Name: query, Conds: args, On: on})
+	return
+}
+
 // Group specify the group method on the find
 func (db *DB) Group(name string) (tx *DB) {
 	tx = db.getInstance()

statement.go

@@ -50,6 +50,7 @@ type Statement struct {
 type join struct {
 	Name  string
 	Conds []interface{}
+	On    clause.Expression
 }
 
 // StatementModifier statement modifier interface