Use Constraint instance in ModelSQL._sql_constraints

issue4814
review15491002

statement.py

@@ -7,7 +7,7 @@ from itertools import groupby
 from sql import Null
 from sql.aggregate import Max, Sum
 
-from trytond.model import Workflow, ModelView, ModelSQL, fields
+from trytond.model import Workflow, ModelView, ModelSQL, fields, Check
 from trytond.pyson import Eval, If, Bool
 from trytond.transaction import Transaction
 from trytond import backend
@@ -558,8 +558,9 @@ class Line(ModelSQL, ModelView):
                 'amount_greater_invoice_amount_to_pay': ('Amount "%s" is '
                     'greater than the amount to pay of invoice.'),
                 })
+        t = cls.__table__()
         cls._sql_constraints += [
-            ('check_statement_line_amount', 'CHECK(amount != 0)',
+            ('check_statement_line_amount', Check(t, t.amount != 0),
                 'Amount should be a positive or negative value.'),
             ]