Rubyonrails has_many 通过大小验证

You can just validate it in the controller, count the phones and render a flash error, before you actually try and save the records.
Doing this in callbacks is difficult and not foolproof.

我添加了一些快速测试，以涵盖创建手机的不同方式:

# spec/models/homes_phone_spec.rb

require "rails_helper"

RSpec.describe HomesPhone, type: :model do
  it "saves 3 new phones" do
    home = Home.create(phones: 3.times.map { Phone.new })
    expect(home.phones.count).to eq 3
  end

  it "doesn't save 4 new phones" do
    home = Home.create(phones: 4.times.map { Phone.new })
    expect(home.phones.count).to eq 0
    expect(home.phones.size).to eq 4
  end

  it "can create up to 3 three phones through association" do
    home = Home.create!
    expect do
      5.times { home.phones.create }
    end.to change(home.phones, :count).by(3)
  end

  it "doesn't add 4th phone to existing record" do
    Home.create(phones: 3.times.map { Phone.new })
    home = Home.last

    # NOTE: every time you call `valid?` or `validate`, it runs validations 
    #       again and all previous errors are cleared.
    # expect(home.phones.create).to be_invalid
    
    expect(home.phones.create).to be_new_record
    # or
    # expect(home.phones.create).not_to be_persisted
    # expect(home.phones.create.errors.any?).to be true
  end

  it "adds phone limit validation error to Phone" do
    home = Home.create(phones: 3.times.map { Phone.new })
    phone = home.phones.create
    expect(phone.errors[:base]).to eq ["too many phones"]
  end
end

# app/models/phone.rb
class Phone < ApplicationRecord
  has_many :homes_phones, dependent: :destroy
  has_many :homes, through: :homes_phones
end

# app/models/home.rb
class Home < ApplicationRecord
  has_many :homes_phones, dependent: :destroy
  has_many :phones, through: :homes_phones
end

# app/models/homes_phone.rb
class HomesPhone < ApplicationRecord
  belongs_to :home
  # NOTE: setting `inverse_of` option will stop raising errors and
  #       just return `phone` with errors that we'll add below
  belongs_to :phone, inverse_of: :homes_phones

  # because of the way through association is saved with `save!` call
  # it raises validation errors. `inverse_of` allows the through association
  # to be properly built and it skips `save!` call:
  # https://github.com/rails/rails/blob/v7.0.4.2/activerecord/lib/active_record/associations/has_many_through_association.rb#L79-L80

  validate do
    # NOTE: if you use `homes_phones` association, the `size` method returns
    #       current count, instead of a lagging `phones.size` count.
    if home.homes_phones.size > 3
      errors.add(:base, "too many phones")
      phone.errors.merge!(errors)
    end
  end
end

$ rspec spec/models/homes_phone_spec.rb

HomesPhone
  saves 3 new phones
  doesn't save 4 new phones
  can create up to 3 three phones through association
  doesn't add 4th phone to existing record
  adds phone limit validation to Phone

Finished in 0.10967 seconds (files took 2.35 seconds to load)
5 examples, 0 failures

但它并不涵盖所有内容，比如:

it "can append up to 3 three phones" do
  home = Home.create!
  expect do
    5.times { home.phones << Phone.new }
  end.to change(home.phones, :count).by(3)
end

$ rspec spec/models/homes_phone_spec.rb:38
Run options: include {:locations=>{"./spec/models/homes_phone_spec.rb"=>[38]}}

HomesPhone
  can append up to 3 three phones (FAILED - 1)

我以为我把一切都修好了.您可以try 执行以下操作:

after_validation do
  if home.homes_phones.size > 3
    errors.add(:base, "too many phones")
    phone.errors.merge! errors
    raise ActiveRecord::Rollback
  end
end

$ rspec spec/models/homes_phone_spec.rb -f p
......

Finished in 0.12411 seconds (files took 2.25 seconds to load)
6 examples, 0 failures