From 895c1178a0d1d837cd986c45eac62f6b10a6add4 Mon Sep 17 00:00:00 2001
From: Adrien Carreira <adrien.carreira@ovhcloud.com>
Date: Thu, 8 Jul 2021 10:04:40 +0200
Subject: [PATCH] Proposal, Add Specific on for Joins queries

---
 callbacks/query.go | 47 ++++++++++++++++++++++++++--------------------
 chainable_api.go   |  6 ++++++
 statement.go       |  1 +
 3 files changed, 34 insertions(+), 20 deletions(-)

diff --git a/callbacks/query.go b/callbacks/query.go
index 3299d015..e5f1250c 100644
--- a/callbacks/query.go
+++ b/callbacks/query.go
@@ -125,33 +125,40 @@ func BuildQuerySQL(db *gorm.DB) {
 						})
 					}
 
-					exprs := make([]clause.Expression, len(relation.References))
-					for idx, ref := range relation.References {
-						if ref.OwnPrimaryKey {
-							exprs[idx] = clause.Eq{
-								Column: clause.Column{Table: clause.CurrentTable, Name: ref.PrimaryKey.DBName},
-								Value:  clause.Column{Table: tableAliasName, Name: ref.ForeignKey.DBName},
-							}
-						} else {
-							if ref.PrimaryValue == "" {
+					if join.On != nil {
+						exprs := make([]clause.Expression, len(relation.References))
+						for idx, ref := range relation.References {
+							if ref.OwnPrimaryKey {
 								exprs[idx] = clause.Eq{
-									Column: clause.Column{Table: clause.CurrentTable, Name: ref.ForeignKey.DBName},
-									Value:  clause.Column{Table: tableAliasName, Name: ref.PrimaryKey.DBName},
+									Column: clause.Column{Table: clause.CurrentTable, Name: ref.PrimaryKey.DBName},
+									Value:  clause.Column{Table: tableAliasName, Name: ref.ForeignKey.DBName},
 								}
 							} else {
-								exprs[idx] = clause.Eq{
-									Column: clause.Column{Table: tableAliasName, Name: ref.ForeignKey.DBName},
-									Value:  ref.PrimaryValue,
+								if ref.PrimaryValue == "" {
+									exprs[idx] = clause.Eq{
+										Column: clause.Column{Table: clause.CurrentTable, Name: ref.ForeignKey.DBName},
+										Value:  clause.Column{Table: tableAliasName, Name: ref.PrimaryKey.DBName},
+									}
+								} else {
+									exprs[idx] = clause.Eq{
+										Column: clause.Column{Table: tableAliasName, Name: ref.ForeignKey.DBName},
+										Value:  ref.PrimaryValue,
+									}
 								}
 							}
 						}
+						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}},
+						})
 					}
-
-					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{
 						Expression: clause.NamedExpr{SQL: join.Name, Vars: join.Conds},
diff --git a/chainable_api.go b/chainable_api.go
index 88279044..32943a83 100644
--- a/chainable_api.go
+++ b/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()
diff --git a/statement.go b/statement.go
index 93b78c12..89824bc1 100644
--- a/statement.go
+++ b/statement.go
@@ -50,6 +50,7 @@ type Statement struct {
 type join struct {
 	Name  string
 	Conds []interface{}
+	On    clause.Expression
 }
 
 // StatementModifier statement modifier interface