diff --git a/odoo/addons/test_testing_utilities/tests/test_form_impl.py b/odoo/addons/test_testing_utilities/tests/test_form_impl.py
index af49d27cbe5..ab846bda87e 100644
--- a/odoo/addons/test_testing_utilities/tests/test_form_impl.py
+++ b/odoo/addons/test_testing_utilities/tests/test_form_impl.py
@@ -374,13 +374,13 @@ class TestO2M(TransactionCase):
 
         with f.subs.edit(index=0) as s:
             self.assertEqual(s.v, 5)
-            self.assertEqual(s.value, False)
+            self.assertEqual(s.value, 2)
 
         r = f.save()
 
         self.assertEqual(
             [get(s) for s in r.subs],
-            [("5", 0, 5)]
+            [("5", 2, 5)]
         )
 
     def test_o2m_inner_default(self):
diff --git a/odoo/models.py b/odoo/models.py
index d82806b6862..f3a2b4d7530 100644
--- a/odoo/models.py
+++ b/odoo/models.py
@@ -5857,6 +5857,10 @@ Fields:
                 names (in view order), or False
             :param field_onchange: dictionary mapping field names to their
                 on_change attribute
+
+            When ``field_name`` is falsy, the method first adds default values
+            to ``values``, applies onchange methods to them, and return all the
+            fields in ``field_onchange``.
         """
         # this is for tests using `Form`
         self.flush()
@@ -5869,6 +5873,8 @@ Fields:
         else:
             names = []
 
+        first_call = not names
+
         if not all(name in self._fields for name in names):
             return {}
 
@@ -5931,14 +5937,16 @@ Fields:
                     )
                 )
 
-            def diff(self, other):
+            def diff(self, other, force=False):
                 """ Return the values in ``self`` that differ from ``other``.
                     Requires record cache invalidation for correct output!
                 """
                 record = self['<record>']
                 result = {}
                 for name, subnames in self['<tree>'].items():
-                    if (name == 'id') or (other.get(name) == self[name]):
+                    if name == 'id':
+                        continue
+                    if not force and other.get(name) == self[name]:
                         continue
                     field = record._fields[name]
                     if field.type not in ('one2many', 'many2many'):
@@ -5968,6 +5976,16 @@ Fields:
 
         nametree = PrefixTree(self.browse(), field_onchange)
 
+        if first_call:
+            names = list(nametree)
+            values.update(self.default_get([
+                name
+                for name in names
+                if name not in values
+            ]))
+            for name in names:
+                values.setdefault(name, False)
+
         # prefetch x2many lines without data (for the initial snapshot)
         for name, subnames in nametree.items():
             if subnames and values.get(name):
@@ -6065,7 +6083,7 @@ Fields:
 
         # determine values that have changed by comparing snapshots
         self.invalidate_cache()
-        result['value'] = snapshot1.diff(snapshot0)
+        result['value'] = snapshot1.diff(snapshot0, force=first_call)
 
         # format warnings
         warnings = result.pop('warnings')
diff --git a/odoo/tests/common.py b/odoo/tests/common.py
index 066bb067148..106cde6f74e 100644
--- a/odoo/tests/common.py
+++ b/odoo/tests/common.py
@@ -1695,42 +1695,12 @@ class Form(object):
 
     def _init_from_defaults(self, model):
         vals = self._values
-        fields = self._view['fields']
-        def cleanup(k, v):
-            if fields[k]['type'] == 'one2many':
-                return [
-                    # use None as "empty" value for UPDATE instead of {}
-                    (1, c[1], None) if c[0] == 1 and not c[2] else c
-                    for c in v
-                    if c[0] != 6 # o2m default gets a (6) at the start, nonsensical
-                ]
-            elif fields[k]['type'] == 'datetime' and isinstance(v, datetime):
-                return odoo.fields.Datetime.to_string(v)
-            elif fields[k]['type'] == 'date' and isinstance(v, date):
-                return odoo.fields.Datetime.to_string(v)
+        vals.clear()
+        vals['id'] = False
 
-            return v
-        defaults = {
-            k: cleanup(k, v)
-            for k, v in model.default_get(list(fields)).items()
-            if k in fields
-        }
-        vals.update(defaults)
-        # m2m should all be rep'd as command list
-        for k, v in vals.items():
-            if not v:
-                type_ = fields[k]['type']
-                if type_ == 'many2many':
-                    vals[k] = [(6, False, [])]
-                elif type_ == 'one2many':
-                    vals[k] = []
-                elif type_ in ('integer', 'float'):
-                    vals[k] = 0
-
-        # on creation, every field is considered changed by the client
-        # apparently
-        # and fields should be sent in view order, not whatever fields_view_get['fields'].keys() is
-        self._perform_onchange(self._view['fields_ordered'])
+        # call onchange with an empty list of fields; this retrieves default
+        # values, applies onchanges and return the result
+        self._perform_onchange([])
 
     def _init_from_values(self, values):
         self._values.update(
@@ -1929,6 +1899,9 @@ class Form(object):
         """
         values = {}
         for f in fields:
+            if f == 'id':
+                continue
+
             get_modifier = functools.partial(
                 self._get_modifier,
                 f, modmap=view['modifiers'],
@@ -1941,8 +1914,8 @@ class Form(object):
                 if get_modifier('required'):
                     raise AssertionError("{} is a required field ({})".format(f, view['modifiers'][f]))
 
-            # skip unmodified fields unless all_fields (also always ignore id)
-            if f == 'id' or not (all_fields or f in changed):
+            # skip unmodified fields unless all_fields
+            if not (all_fields or f in changed):
                 continue
 
             if get_modifier('readonly'):
@@ -1997,7 +1970,7 @@ class Form(object):
         # skip calling onchange() if there's no trigger on any of the changed
         # fields
         spec = self._view['onchange']
-        if not any(spec[f] for f in fields):
+        if fields and not any(spec[f] for f in fields):
             return
 
         record = self._model.browse(self._values.get('id'))
@@ -2012,7 +1985,7 @@ class Form(object):
         self._values.update(
             (k, self._cleanup_onchange(
                 self._view['fields'][k],
-                v, self._values[k],
+                v, self._values.get(k),
             ))
             for k, v in values.items()
             if k in self._view['fields']