Add foreign key for relation of model field

Closes #9050

trytond/trytond/ir/model.py

@@ -249,6 +249,9 @@ class ModelField(
             'module_ref', 'module', 'ir.module,name', "Module",
             readonly=True, ondelete='CASCADE',
             help="Module in which this field is defined."),
+        fields.fmany2one(
+            'relation_ref', 'relation', 'ir.model,name', "Relation",
+            ondelete='CASCADE'),
         ModelSQL, ModelView):
     __name__ = 'ir.model.field'
     _rec_name = 'string'
@@ -334,6 +337,7 @@ class ModelField(
 
     @classmethod
     def register(cls, model, module_name, model_id):
+        pool = Pool()
         cursor = Transaction().connection.cursor()
 
         ir_model_field = cls.__table__()
@@ -358,6 +362,11 @@ class ModelField(
                 relation = field.relation_name
             else:
                 relation = None
+
+            if relation:
+                Relation = pool.get(relation)
+                Model.register(Relation, module_name)
+
             access = field_name in model.__access__
 
             if field_name not in model_fields:

trytond/trytond/ir/view/model_field_form.xml

@@ -13,8 +13,9 @@ this repository contains the full copyright notices and license terms. -->
 
     <label name="ttype"/>
     <field name="ttype"/>
-    <label name="relation"/>
-    <field name="relation"/>
+    <label name="relation_ref"/>
+    <field name="relation_ref"/>
+
     <label name="help" yfill="1"/>
     <field name="help" colspan="3"/>
     <label name="access"/>

trytond/trytond/ir/view/model_field_list.xml

@@ -6,7 +6,7 @@ this repository contains the full copyright notices and license terms. -->
     <field name="name" expand="1" optional="1"/>
     <field name="model_ref" expand="1"/>
     <field name="ttype" optional="0"/>
-    <field name="relation" expand="1" optional="1"/>
+    <field name="relation_ref" expand="1" optional="1"/>
     <field name="access" optional="1"/>
     <field name="module_ref" optional="1"/>
 </tree>