diff --git a/odoo/addons/test_new_api/models/test_new_api.py b/odoo/addons/test_new_api/models/test_new_api.py
index 602173c4145..99f0fd65b68 100644
--- a/odoo/addons/test_new_api/models/test_new_api.py
+++ b/odoo/addons/test_new_api/models/test_new_api.py
@@ -244,7 +244,7 @@ class MultiLine(models.Model):
 
     multi = fields.Many2one('test_new_api.multi', ondelete='cascade')
     name = fields.Char()
-    partner = fields.Many2one('res.partner')
+    partner = fields.Many2one(related='multi.partner', store=True)
     tags = fields.Many2many('test_new_api.multi.tag')
 
 
diff --git a/odoo/addons/test_new_api/tests/test_onchange.py b/odoo/addons/test_new_api/tests/test_onchange.py
index 5bffeae5f86..38c7b8be531 100644
--- a/odoo/addons/test_new_api/tests/test_onchange.py
+++ b/odoo/addons/test_new_api/tests/test_onchange.py
@@ -378,6 +378,19 @@ class TestOnChange(SavepointCaseWithUserDemo):
         result = Foo.onchange(values, 'value1', field_onchange)
         self.assertEqual(result['value'], {})
 
+    def test_onchange_one2many_first(self):
+        partner = self.env['res.partner'].create({
+            'name': 'X',
+            'country_id': self.env.ref('base.be').id,
+        })
+        with common.Form(self.env['test_new_api.multi']) as form:
+            form.partner = partner
+            self.assertEqual(form.partner, partner)
+            self.assertEqual(form.name, partner.name)
+            with form.lines.new() as line:
+                # the first onchange() must have computed partner
+                self.assertEqual(line.partner, partner)
+
     def test_onchange_one2many_value(self):
         """ test the value of the one2many field inside the onchange """
         discussion = self.env.ref('test_new_api.discussion_0')
diff --git a/odoo/fields.py b/odoo/fields.py
index 46d65d4e6f4..2838bfbef56 100644
--- a/odoo/fields.py
+++ b/odoo/fields.py
@@ -963,7 +963,14 @@ class Field(MetaField('DummyField', (object,), {})):
                         self.compute_value(recs)
                     except (AccessError, MissingError):
                         self.compute_value(record)
-                    value = env.cache.get(record, self)
+                    try:
+                        value = env.cache.get(record, self)
+                    except CacheMiss:
+                        if self.readonly:
+                            raise
+                        # fallback to null value if compute gives nothing
+                        value = self.convert_to_cache(False, record, validate=False)
+                        env.cache.set(record, self, value)
 
             elif self.type == 'many2one' and self.delegate and not record.id:
                 # parent record of a new record: new record, with the same
diff --git a/odoo/models.py b/odoo/models.py
index f3a2b4d7530..991573a4155 100644
--- a/odoo/models.py
+++ b/odoo/models.py
@@ -5859,8 +5859,8 @@ Fields:
                 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``.
+            to ``values``, computes the remaining fields, applies onchange
+            methods to them, and return all the fields in ``field_onchange``.
         """
         # this is for tests using `Form`
         self.flush()
@@ -5902,11 +5902,12 @@ Fields:
             """ A dict with the values of a record, following a prefix tree. """
             __slots__ = ()
 
-            def __init__(self, record, tree):
+            def __init__(self, record, tree, fetch=True):
                 # put record in dict to include it when comparing snapshots
                 super(Snapshot, self).__init__({'<record>': record, '<tree>': tree})
-                for name in tree:
-                    self.fetch(name)
+                if fetch:
+                    for name in tree:
+                        self.fetch(name)
 
             def fetch(self, name):
                 """ Set the value of field ``name`` from the record's value. """
@@ -5920,6 +5921,8 @@ Fields:
 
             def has_changed(self, name):
                 """ Return whether a field on record has changed. """
+                if name not in self:
+                    return True
                 record = self['<record>']
                 subnames = self['<tree>'][name]
                 if record._fields[name].type not in ('one2many', 'many2many'):
@@ -5977,14 +5980,11 @@ Fields:
         nametree = PrefixTree(self.browse(), field_onchange)
 
         if first_call:
-            names = list(nametree)
             values.update(self.default_get([
                 name
-                for name in names
+                for name in nametree
                 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():
@@ -6032,7 +6032,7 @@ Fields:
         record = self.new(initial_values, origin=self)
 
         # make a snapshot based on the initial values of record
-        snapshot0 = Snapshot(record, nametree)
+        snapshot0 = Snapshot(record, nametree, fetch=(not first_call))
 
         # store changed values in cache; also trigger recomputations based on
         # subfields (e.g., line.a has been modified, line.b is computed stored