function m2mToFieldName(otherModelName) {
    return `to${otherModelName}Id`;
}
Returns the fieldname that saves a foreign key in a many-to-many through model to the
model where the many-to-many relation was declared.

Example: `Book` => `toBookId`

@param  {string} otherModelName - the name of the model that was the target of the many-to-many
                                  declaration.
@return {string} the field name in the through model for `otherModelName`'s foreign key..