diff --git a/.drone.yml b/.drone.yml
index a52ef0e..b873d8e 100644
--- a/.drone.yml
+++ b/.drone.yml
@@ -7,7 +7,8 @@ pipeline:
         image: ${IMAGE}
         environment:
             - CFLAGS=-O0
-            - TOX_TESTENV_PASSENV=CFLAGS
+            - DB_CACHE=/cache
+            - TOX_TESTENV_PASSENV=CFLAGS DB_CACHE
             - POSTGRESQL_URI=postgresql://postgres@postgresql:5432/
         commands:
             - pip install tox
diff --git a/__init__.py b/__init__.py
index 8052da2..04573fa 100644
--- a/__init__.py
+++ b/__init__.py
@@ -1,10 +1,11 @@
 # The COPYRIGHT file at the top level of this repository contains the full
 # copyright notices and license terms.
 from trytond.pool import Pool
-from .party import Party
+from .party import Party, PartyParty
 
 
 def register():
     Pool.register(
         Party,
+        PartyParty,
         module='account_invoice_party_invoice_to', type_='model')
diff --git a/party.py b/party.py
index 913ca5c..5fd0caa 100644
--- a/party.py
+++ b/party.py
@@ -1,16 +1,31 @@
 # The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
-from trytond.model import fields
-from trytond.pool import PoolMeta
+from trytond.model import ModelSQL, fields
+from trytond.pool import PoolMeta, Pool
+from trytond.modules.company.model import CompanyValueMixin
 
-__all__ = ['Party']
+__all__ = ['Party', 'PartyParty']
 
 
 class Party:
     __name__ = 'party.party'
     __metaclass__ = PoolMeta
 
-    customer_to_invoice = fields.Property(
+    customer_to_invoice = fields.MultiValue(
             fields.Many2One('party.party', 'Customer to invoice'))
-    supplier_to_invoice = fields.Property(
+    supplier_to_invoice = fields.MultiValue(
             fields.Many2One('party.party', 'Supplier to invoice'))
+
+    @classmethod
+    def multivalue_model(cls, field):
+        pool = Pool()
+        if field in {'customer_to_invoice', 'supplier_to_invoice'}:
+            return pool.get('party.party.party')
+        return super(Party, cls).multivalue_model(field)
+
+
+class PartyParty(ModelSQL, CompanyValueMixin):
+    "Party Party"
+    __name__ = 'party.party.party'
+    customer_to_invoice = fields.Many2One('party.party', 'Customer to invoice')
+    supplier_to_invoice = fields.Many2One('party.party', 'Supplier to invoice')
diff --git a/tox.ini b/tox.ini
index d205768..3d844ea 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,6 +7,7 @@ deps =
     {py27,py34,py35,py36}-postgresql: psycopg2 >= 2.5
     pypy-postgresql: psycopg2cffi >= 2.5
     mysql: MySQL-python
+    sqlite: sqlitebck
 setenv =
     sqlite: TRYTOND_DATABASE_URI={env:SQLITE_URI:sqlite://}
     postgresql: TRYTOND_DATABASE_URI={env:POSTGRESQL_URI:postgresql://}
diff --git a/tryton.cfg b/tryton.cfg
index 7cb7338..f8ebdff 100644
--- a/tryton.cfg
+++ b/tryton.cfg
@@ -1,5 +1,5 @@
 [tryton]
-version=4.2.0
+version=4.8.0
 depends:
   account_invoice
   ir