Set shipment/invoice method as configuration option

configuration.py

@@ -1,7 +1,7 @@
 #This file is part of Tryton.  The COPYRIGHT file at the top level of
 #this repository contains the full copyright notices and license terms.
 from trytond.model import ModelView, ModelSQL, ModelSingleton, fields
-from trytond.pyson import Eval
+from trytond.pyson import Eval, Bool
 
 
 class Configuration(ModelSingleton, ModelSQL, ModelView):
@@ -14,5 +14,12 @@ class Configuration(ModelSingleton, ModelSQL, ModelView):
             ('company', 'in', [Eval('company'), False]),
             ('code', '=', 'purchase.purchase'),
         ], required=True))
+    purchase_invoice_method = fields.Property(fields.Selection([
+        ('manual', 'Manual'),
+        ('order', 'Based On Order'),
+        ('shipment', 'Based On Shipment'),
+    ], 'Invoice Method', states={
+        'required': Bool(Eval('company')),
+    }))
 
 Configuration()

configuration.xml

@@ -11,6 +11,8 @@ this repository contains the full copyright notices and license terms. -->
                 <form string="Purchase Configuration">
                     <label name="purchase_sequence"/>
                     <field name="purchase_sequence"/>
+                    <label name="purchase_invoice_method" />
+                    <field name="purchase_invoice_method" />
                 </form>
                 ]]>
             </field>
@@ -36,5 +38,12 @@ this repository contains the full copyright notices and license terms. -->
                 search="[('model.model', '=', 'purchase.configuration'), ('name', '=', 'purchase_sequence')]"/>
             <field name="value" eval="'ir.sequence,' + str(ref('sequence_purchase'))"/>
         </record>
+        <record model="ir.property"
+            id="property_purchase_invoice_method">
+            <field name="name">purchase_invoice_method</field>
+            <field name="field"
+               search="[('model.model', '=', 'purchase.configuration'), ('name', '=', 'purchase_invoice_method')]" />
+           <field name="value">,order</field>
+        </record>
     </data>
 </tryton>

purchase.py

@@ -187,7 +187,9 @@ class Purchase(ModelWorkflow, ModelSQL, ModelView):
         return 2
 
     def default_invoice_method(self):
-        return 'order'
+        configuration_obj = self.pool.get('purchase.configuration')
+        configuration = configuration_obj.browse(1)
+        return configuration.purchase_invoice_method
 
     def default_invoice_state(self):
         return 'none'