allow same partner on both sides of a relation

* add boolean field `ResPartnerRelationType.allow_self`
* add related field `ResPartnerRelation.allow_self`

partner_relations/model/res_partner_relation.py

@@ -82,6 +82,7 @@ class ResPartnerRelation(models.Model):
         ),
     }
 
+    allow_self = fields.Boolean(related='type_id.allow_self')
     left_contact_type = fields.Selection(
         lambda s: s.env['res.partner.relation.type']._get_partner_types(),
         'Left Partner Type',
@@ -287,9 +288,10 @@ class ResPartnerRelation(models.Model):
         :raises exceptions.Warning: When constraint is violated
         """
         if self.left_partner_id == self.right_partner_id:
-            raise exceptions.Warning(
-                _('Partners cannot have a relation with themselves.')
-            )
+            if not self.allow_self:
+                raise exceptions.Warning(
+                    _('Partners cannot have a relation with themselves.')
+                )
 
     @api.one
     @api.constrains('left_partner_id', 'right_partner_id', 'active')

partner_relations/model/res_partner_relation_type.py

@@ -57,6 +57,9 @@ class ResPartnerRelationType(models.Model):
         'res.partner.category',
         'Right partner category',
     )
+    allow_self = fields.Boolean(
+        'Allow both sides to be the same'
+    )
 
     @api.model
     def _get_partner_types(self):

partner_relations/view/res_partner_relation_type.xml

@@ -9,6 +9,7 @@
                     <field name="name_inverse" />
                     <field name="contact_type_left" />
                     <field name="contact_type_right" />
+                    <field name="allow_self" />
                 </tree>
             </field>
         </record>
@@ -19,6 +20,7 @@
                 <form version="7.0" string="Partner relation">
                     <sheet>
                         <group>
+                            <field name="allow_self" />
                             <group
                                 colspan="2" col="2"
                                 string="Left side of relation"