How to specify the association with factory_bot?

Here we have another solution in case your association name and factory name is different then you can follow below syntax.

#spec/factories/post.rb
FactoryBot.define do
  factory :post do
    association :author, factory: :user
    body Faker::Movie.quote
    posted_at "2018-04-03 13:33:05"
  end
end

in case your factory name is author and model name is user then you can use above syntax

Another option is to use #trait method within the parent.

FactoryBot.define do
  factory :post do
    user nil
    body Faker::Movie.quote
    posted_at "2018-04-03 13:33:05"
  end
end

FactoryBot.define do 
  factory :user do 
    first_name 'Jake'
  end

  trait :with_post do
    after(:create) do |user|
      create(:post, user_id: user.id)
    end
  end
end

FactoryBot.create(:user, :with_post)

You should use explicit associations instead of implicit ones:

#spec/factories/post.rb
FactoryBot.define do
  factory :post do
    association :user    # <<<--- here the change
    body Faker::Movie.quote
    posted_at "2018-04-03 13:33:05"
  end
end

#spec/factories/user.rb
FactoryBot.define do 
  factory :user do 
    first_name 'Jake'
  end
end

https://github.com/thoughtbot/factory_bot/blob/master/GETTING_STARTED.md#associations