explanation of odoo filter domain operator "child_of" and its preference over "in" operator

Question

I am uncertain regarding what child_of does after reading the docs and the use of child_of operator over using in operator for filtering

Odoo v10 domain docs can be found here which states the following for child_of

is a child (descendant) of a value record.
Takes the semantics of the model into account (i.e following the relationship field named by _parent_name)

Consider the model product.product having a many2one field called pos_categ_id that points to model pos.category

To only get products with PoS category id of x, domain value of ['pos_categ_id', 'child_of', x] can be used, however ['pos_categ_id', 'in', [x]] seems to also do the same thing.

In addition to that, in operator can be used for many2many and one2many fields with same format as above example while use of child_of operator for those two field types cause errors.

Example the domain ['session_ids', 'child_of', [572]] where session_ids is a many2many field causes the following error Invalid field 'parent_id' in leaf \"<osv.ExtendedLeaf: ('parent_id', 'in', [572]) on pos_session (ctx: )>\"

So, what is the example case where child_of is preferred over in operator? Both seems to function the same except in can work on all types of relational fields. Second, what does the second line of docs for child_of mean by the phrase following the relationship field named by _parent_name?

Answer 1

No they are not the same, yes they are the same in the simple example that you used:

To only get products with PoS category id of x, domain value of ['pos_categ_id', 'child_of', x] can be used, however ['pos_categ_id', 'in', [x]] seems to also do the same thing.

But consider this case:

#               x                
#              / \               
#             f   b                
#                / \               
#               c   d  

when we do this ['pos_categ_id', 'child_of', x] this will return product that their category is one of this list [x, f, b, c, d] it's equivalent to ['pos_categ_id', 'in', [x, f, b, c, d]]. because they are all child of x (child_of includes x itself).

but this: ['pos_categ_id', 'in', [x]] will return only product that their category is X but will not return product with category b or c ..etc.

To activate this feature in your custom model you should set this attribute

  class YourModel(models.Model):
     _name = 'some.model'
     _parent_name = 'parent_id'  # by default its name is parent_id you can change it
     _parent_store = True  # tell odoo that this model support parent & child relation ship

     # same name of _parent_name 
     parent_id = fields.Many2one('some.model', 'Parent')

Answer 2

Basically child_of functionality is provided by odoo, not python.

But to understand it you must need to knowledge of parent, 'child` relationship in database concept.

If any of your tables are using a self join like below example.

_name = 'my.table'

parent_id = fields.Many2one('my.table', string='Parent Id')

In the above case, your model has connected itself. In this type of cases in Odoo, you can use child_of operator in the domain.

So, it will search in database like parent_id = <your_value>.

For in domain,

You can pass the list of data which you need to search in id field. It similar to any database in operator.