Add a new relationship between the parent and child tables.
The relationship is created by adding a reference (ref) on the foreign_key
field of the child table pointing at the parent primary key.
parent (str) – Name of the parent table.
child (str) – Name of the child table.
foreign_key (str) – Field in the child table through which the relationship is created.
If None, use the parent primary key name.
validate (bool) – Whether to validate metadata after adding this relationship or not.
Defaults to True.
ValueError – If any of the following happens:
* The parent table does not exist.
* The child table does not exist.
* The parent table does not have a primary key.
* The foreign_key field already exists in the child table.
* The child table already has a parent.
* The new relationship closes a relationship circle.