Hardcode constraints in the `create` method instead of using normal ones.

Depending on the context where the record is created, it gets `resource` or `model_id`. The problem is that Odoo checks constrains before inverses, so constrains would fail always.

Test added to ensure future versions contemplate all use cases.

base_export_manager/views/ir_exports_view.xml

@@ -1,4 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright 2016 Jairo Llopis <jairo.llopis@tecnativa.com>
+     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->
 <openerp>
 <data>
 
@@ -38,7 +40,8 @@
                 <field name="resource"/>
             </group>
             <group>
-                <field name="export_fields" nolabel="1">
+                <field name="export_fields" nolabel="1"
+                       context="{'default_model1_id': model_id}">
                     <tree editable="bottom">
                         <field name="sequence" invisible="True"/>
                         <field name="model1_id" invisible="True"/>