Why i am getting this error in Factory: must be exist card? how can i solved it?

Only i want to create a 'post' but i am getting this error:

Failure/Error: let!(:post) { FactoryBot.create(:post, company_id: company.id, card_id: card.id) } 
ActiveRecord::RecordInvalid:
        Validation failed: must be exist card

the model of post is:

class Post < ApplicationRecord
  validates :first_title, :sub_title, :email, presence: true
  belongs_to :card
  delegate :max, to: :card
end

and card:

class Card < ApplicationRecord
  include Discard::Model
end

factories:

factory :post do
    first_title { Faker::Name.name }
    sub_title { Faker::Name.name }
    email { Faker::Internet.email}
    card_id {}
  end

factory for card:

factory :card do
  company_id {}
end

It seems your factories are not well defined. This should be the first thing to make sure it works fine. Try to define "associations" like this:

factory :post do
  first_title { Faker::Name.name }
  sub_title { Faker::Name.name }
  email { Faker::Internet.email }
  card
end

Or:

factory :post do
  first_title { Faker::Name.name }
  sub_title { Faker::Name.name }
  email { Faker::Internet.email }
  card { FactoryBot.create(:card) }
end

Or also, you can try using the association macro:

factory :post do
  first_title { Faker::Name.name }
  sub_title { Faker::Name.name }
  email { Faker::Internet.email}
  association :card, factory: :card
end

Compiling the response from the other question:

factory :card do
  association :company, factory: :company
end

factory :post do
  first_title { Faker::Name.name }
  sub_title { Faker::Name.name }
  email { Faker::Internet.email}

  transient do
    company_id { FactoryBot.create(:company).id }
  end

  after(:build) do |post, evaluator|
    card = FactoryBot.create(:card, company_id: evaluator.company_id)
    post.card = card
  end
end

let(:company) { FactoryBot.create :company }
let!(:post) { FactoryBot.create(:post, company_id: company.id }
let(:card) { post.card }