[RUBY] Existence check of has_many and belongs_to-optional: What is true?

Motivation

I think that it is necessary to check the existence of related destinations on the belongs_to side in many cases, but I did not encounter many related destinations on the has_many side, so I checked it.

Thing you want to do

Association of user model and team model. However, with the following restrictions.

--User must have at least one team. (Must belong to one or more teams) --On the contrary, there should be no team that is not related to user.

↓ This is the association.

environment

Rails 6.0.3 Ruby 2.7.1 mysql2 0.5.3

post model association

class Team < ApplicationRecord
    belongs_to :user
end

This is all you need for the post model. When you write belongs_to, ʻoptional: false` is applied by default. This imposes the constraint "post must always be associated with user".

On the contrary, if you want to create a "post that is not tied to user",

class Team < ApplicationRecord
    belongs_to :user, optional: true
end

It's OK if you write it like this. ʻOptional: true` does not necessarily have to be associated with user.

user model association

class User < ApplicationRecord
    has_many :teams
    validates :teams, presence: true
end

If you want to check the existence of related destinations on the has_many side, you need to add validation. If you do the above, you will be constrained that you must have at least one team.

Finally

I hope it helps someone.