Purpose
=======
mail.tracking.values should be deleted when the corresponding
field is deleted (when the module which defined it is uninstalled).
Firstly because we don't want useless data in db.
Secondly because the groups associated with the field can no longer
be checked if it was deleted. As we don't know to whom the value
was restricted, the value should not be displayed anyway.
Note: this case was fixed in saas-12.2 by b9e96b7 but the proper
way to fix it is to delete the tracking values.
Specification
=============
Delete `mail.tracking.value` when the associated
`ir.model.fields` is deleted.
Two alternatives were considered:
1. Change the `field` field of `mail.tracking.value` from Char
to a m2o to `ir.model.fields` which allows to use delete
oncascade. This implies to modify existing code, but more
importantly it adds database queries.
2. Override the unlink method of `ir.model.fields` to
first unlink associated tracking values.
The first method is probably cleaner but the second method
was nonetheless chosen as we don't want to impact
performance in the main tracking flow only to better
support module uninstalls which happens rarely.
Note: when a module is uninstalled, `ir.model.fields`
are unlinked one by one (in a for loop). Tracking values are
therefore also unlinked field by field. Batchifying field
deletion would greatly reduce the amount of queries.
Lucas Lefèvre
8d2d41068d