Make each company's party unique

modules/company/company.py

@@ -72,6 +72,15 @@ class Company(ModelSQL, ModelView):
         help="Uses the first identifier that matches the criteria.\n"
         "If none match, uses the party's tax identifier.")
 
+    @classmethod
+    def __setup__(cls):
+        super().__setup__()
+        t = cls.__table__()
+        cls._sql_constraints += [
+            ('party_unique', Unique(t, t.party),
+                'company.msg_company_party_unique'),
+            ]
+
     @property
     def header_used(self):
         return Template(self.header or '').safe_substitute(self._substitutions)

modules/company/message.xml

@@ -3,6 +3,9 @@
 this repository contains the full copyright notices and license terms. -->
 <tryton>
     <data>
+        <record model="ir.message" id="msg_company_party_unique">
+            <field name="text">A party can only be assigned to one company.</field>
+        </record>
         <record model="ir.message" id="msg_company_logo_cache_size_unique">
             <field name="text">The size of cached company logo must be unique.</field>
         </record>