Company has many users とします。

class Company < ApplicationRecord
  has_many :users
end

class User < ApplicationRecord
  belongs_to :company
end

会社をひとつつくる

Company.create(name: 'kinop')
=> #<Company:0x00000005e599d0
 id: 1,
 name: "kinop",
 created_at: Mon, 19 Jun 2017 05:44:02 UTC +00:00,
 updated_at: Mon, 19 Jun 2017 05:44:02 UTC +00:00>

Company.count => 1

ユーザを3人つくる

1.upto(3) do |i|
  User.create(name: "name#{i}", company_id: 1)
end

User.count => 3

問題1

company = Company.first
company.users.count # => ?

問題2

company = Company.first
company.users.size # => ?

問題3

company = Company.first
company.users.length # => ?

問題4

company = Company.first
company.users.length # => 3
User.create(name: "name4", company_id:1)
puts "#{company.users.length} #{company.users.size} #{company.users.count}"

解答

問題1 => 3
問題2 => 3
問題3 => 3
問題4 => 3 3 4