Fill the source value of fmany2one in create and write methods

We can not rely on the Function field setter to set the value because it may be
too late if the source field is required and for composite key, a partial
update may not be a valid foreign key.

Closes #13522

trytond/trytond/model/fields/fmany2one.py

@@ -17,6 +17,12 @@ def fmany2one(
     target_fields = target_fields.split(',')
     assert len(sources) == len(target_fields)
 
+    def convert_value(value):
+        pool = Pool()
+        Target = pool.get(target_model)
+        if isinstance(value, int):
+            return getattr(Target(value), target_fields[0])
+
     class Mixin:
         __slots__ = ()
 
@@ -30,6 +36,25 @@ def fmany2one(
                 sources, Target._table, target_fields,
                 on_delete=getattr(cls, name).ondelete)
 
+        @classmethod
+        def create(cls, vlist):
+            vlist = [v.copy() for v in vlist]
+            for values in vlist:
+                if name in values:
+                    values[sources[0]] = convert_value(values.pop(name))
+            return super().create(vlist)
+
+        @classmethod
+        def write(cls, *args):
+            actions = iter(args)
+            args = []
+            for records, values in zip(actions, actions):
+                if name in values:
+                    values = values.copy()
+                    values[sources[0]] = convert_value(values.pop(name))
+                args.extend((records, values))
+            super().write(*args)
+
     @classmethod
     def getter(cls, records, name):
         pool = Pool()
@@ -51,13 +76,7 @@ def fmany2one(
 
     @classmethod
     def setter(cls, records, name, value):
-        pool = Pool()
-        Target = pool.get(target_model)
-        if value:
-            value = getattr(Target(value), target_fields[0])
-        else:
-            value = None
-        cls.write(records, {sources[0]: value})
+        cls.write(records, {sources[0]: convert_value(value)})
 
     @classmethod
     def searcher(cls, clause, tables):