OpenERP Document

doc.openerp.com trunk server 03_module_dev_02

tells that:

To define a new object, you must define a new Python class then instantiate it. This class must inherit from the osv class in the osv module.

and gives a sample code:

class name_of_the_object( osv.osv ):
     _name = 'name.of.the.object'
     _columns = { ... }
    #...
name_of_the_object()  #instantiate here

However, in Product Variant Multi module, objects in product_variant.py like

product_variant_dimension_type

and

product_variant_dimension_option

do not have this kind of code to do instantiation.

class product_variant_dimension_type(orm.Model):
    _name = "product.variant.dimension.type"
    _description = "Dimension Type"
    _columns = {
        # fields...
    }
    # others...
# No instantiate code: "product_variant_dimension_type()" here

So when should we instantiate an object?