are you sure you need to use inheritance by delegation as  _inherits = {'product.template': 'product_tmpl_id'}  ?

try with usual inheritance (a _inherit, NOT the _inherits):

_inherit = 'product.template'

refer to the documentation

Try this friend:

class product_product(osv.osv):

_name = "product.product"

_description = "Product"

_inherit = ['product.template']

_sql_constraints = [

('default_code', 'unique(default_code)', 'Product Reference must be unique!'),

] 

If it doesnt work

try this:

class product_product(osv.osv):

_name = "product.product"

_description = "Product"

_inherit = ['product.template']

_columns = {

'default_code' : fields.char('Internal Reference', select=True),

_sql_constraints = [

('default_code', 'unique(default_code)', 'Product Reference must be unique!'),

]

}

And dont forget to add in your XML:

<record id="view_product_form_inherit" model="ir.ui.view">

<field name="name">product.product.form.inherit</field>

<field name="model">product.product</field>

<field name="inherit_id" ref="product.product_normal_form_view"/>

<xpath expr="//field[@name='default_code']" position="replace">

<field name="default_code"/>

</xpath>

</field>

</record>

</data>

</openerp>

PS : Pay attention to the model and name of view in your XML.

Hello,

According to the below code Odoo will add one unique constraint inside postgresql database.

class product_product(osv.osv):
    _name = "product.product"
    _description = "Product"
    _inherit = 'product.product'

    _sql_constraints = [
        ('default_code', 'unique(default_code)', 'Product Reference must be unique!'),
    ]
product_product() 

But according to the already available records there may be some situations like :

1) When all records have default_code is already unique : In this case when Odoo is going to add unique constraint on the field default_code at that time inside database one constraint is added for the field default_code inside product_product table, and it is added successfully. After that you can also see the effect of that constraint and you can not make any duplicate value inside default_code field.

2) When all records have default_code is not unique / Already there are some duplicate value is there : In this case when Odoo is going to add unique constraint on the field default_code at that time inside database it is not possible to apply unique constraint on already duplicated data. So, at that time constraint will not perfectly done at database side and Odoo also give us WARNING :

"""Table 'product_product': unable to add 'unique(default_code)' constraint ! If you want to have it, you should update the records and execute manually:  ALTER TABLE "product_product" ADD CONSTRAINT "product_product_default_code" unique(default_code)"""  

So, in this case if we want to apply this constraint first of all we have to make changes inside database to do so all default_code value is unique and then after just upgrade the module again, Odoo will add that constraint on that field or you can execute manual query as give inside warning.

In your code might be the issue of the data not of code. I hope it is helpful to you for understanding that how _sql_constraint is work in Odoo.

Thanks.