In this case, I am fairly conviced that the foreignKey is not optionnal. Notice that the model actually is User but we’re using sender_id and receiver_id as the FKs.

Cookbook:
foreignKey: the name of the foreign key found in the current model. This is especially handy if you need to define multiple belongsTo relationships. The default value for this key is the underscored, singular name of the other model, suffixed with ‘_id’.