diff --git a/modules/purchase_trade/backtoback.py b/modules/purchase_trade/backtoback.py
index baf25d3..c4f0c30 100644
--- a/modules/purchase_trade/backtoback.py
+++ b/modules/purchase_trade/backtoback.py
@@ -18,5 +18,5 @@ class Backtoback(ModelSQL, ModelView):
     __name__ = 'back.to.back'
 
     reference = fields.Char("Reference")
-    purchase = fields.One2Many('purchase.purchase', "Purchase")
+    purchase = fields.One2Many('purchase.purchase','btb', "Purchase")
     
\ No newline at end of file
diff --git a/modules/purchase_trade/purchase.py b/modules/purchase_trade/purchase.py
index 228e79c..74d9f8c 100755
--- a/modules/purchase_trade/purchase.py
+++ b/modules/purchase_trade/purchase.py
@@ -247,6 +247,7 @@ class PurchaseCrop(ModelSQL,ModelView):
 class Purchase(metaclass=PoolMeta):
     __name__ = 'purchase.purchase'
 
+    btb = fields.Many2One('back.to.back',"Back to back")
     from_location = fields.Many2One('stock.location', 'From location',domain=[('type', "!=", 'customer')])
     to_location = fields.Many2One('stock.location', 'To location',domain=[('type', "!=", 'supplier')])
     shipment_in = fields.Many2One('stock.shipment.in','Purchases')
diff --git a/modules/purchase_trade/sale.py b/modules/purchase_trade/sale.py
index 0537520..8bfcb44 100755
--- a/modules/purchase_trade/sale.py
+++ b/modules/purchase_trade/sale.py
@@ -49,7 +49,7 @@ class Backtoback(metaclass=PoolMeta):
     'Back To Back'
     __name__ = 'back.to.back'
 
-    sale = fields.One2Many('sale.sale', "Sale")
+    sale = fields.One2Many('sale.sale','btb', "Sale")
 
 class OpenPosition(metaclass=PoolMeta):
     "Open position"
@@ -210,6 +210,7 @@ class Lot(metaclass=PoolMeta):
 class Sale(metaclass=PoolMeta):
     __name__ = 'sale.sale'
 
+    btb = fields.Many2One('back.to.back',"Back to back")
     from_location = fields.Many2One('stock.location', 'From location',domain=[('type', "!=", 'customer')])
     to_location = fields.Many2One('stock.location', 'To location',domain=[('type', "!=", 'supplier')])
     shipment_out = fields.Many2One('stock.shipment.out','Sales')