How can I set up a "many to many" relation with an entity

Question

How do I set up n:m database relation in a custom Drupal 8 module?

I want to create task entities that may be related to different users (a task may have multiple related users and a user may have different tasks). Using the database directly I would create two tables(as the user table already exists), a tasks table and a task_users table which contains the relations (User IDs and Task ID)s. How do provide something like this in Drupal 8?

I only find a way to handle 1:n relations in setting up a user reference field (entity_reference) as a base field definition.

$fields['user'] = BaseFieldDefinition::create('entity_reference')

But how do I provide a relationship table?

Answer 1

You can set unlimit cardinality, some like this:

->setCardinality(BaseFieldDefinition::CARDINALITY_UNLIMITED)

This create a intermediate table to reference two entities.

Check api documentation: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Field%21BaseFieldDefinition.php/function/BaseFieldDefinition%3A%3AsetCardinality/8.6.x