Why on_change method with one2many parameter return ids instead of value?

[Randy]

I have two module, one parent and one child modules. Parent module have one2many field to child module. I put on_change method in the one2many field with parameter is the field itself.

In on_change method(.py file) when creating new line in one2many field, the parameter looks like

[(0,0,{value})]

Then, I save the module, and try to edit the one2many field.

It give me

[(6,0,[xx,xy])]

with xx and xy are id's of child.

My question is, when I change some value in child's field, how can parent know the changed value in child's field.

I already tried to browse the record of xx, and xy. It gives me the old value stored in db.

Someone pls enlighten me! Odoo-8.0