Given these classes:

class years(models.Model): 
    _name = 'years'    
    name = fields.Integer(required=True)

For different years I have different categories.

class category(models.Model): 
    _name = 'category' 
    year_id = fields.Many2one('years', required=True, default=get_year)
    name = fields.Char(required=True)

class parent(models.Model):
    _name = 'parent'
    name = fields.Char(required=True)
    year_id = fields.Many2one('years', required=True, default=get_year)

class child(models.Model): 
    _name = 'child' 
    parent_id = fields.Many2one('parent', required=True) 
    name = fields.Char(required=True) 
    year_id = fields.Many2one('years', required=True, default=get_year) 
    category_id = fields.Many2one('category', required=True)
    value = fields.Float(required=True)

The data looks like this:

years

id name
1  2015
2  2016

category

id year_id name
1  1       Sample 1
2  1       Sample 2
3  2       Sample 3
4  2       Sample 4

parent:

id name       year_id
1  Parent 1   1

child should be this:

id parent_id name    year_id category_id value
1  1         Child 1 1       1           154.452   
1  1         Child 2 1       2           100

instead of that I have this:

id parent_id name      year_id category_id value
1  1         Child 1   1       1           154.452 
1  1         Child 2   1       4           100

My question is: How can I "inherit" the year_id in the child record from the parent, and how can I limit the categories in the dropdown list (Many2one) to that specific year?

As I've seen I cannot have a related field parent_id.year_id as during the creation this value is not set so I cannot use a domain restriction of the category to that specific year.

Any idea?