* 1. Define the relationship in the initializer file


  Associations can be defined in the initializer file, and when Arql starts, it first generates the model class based on
  the database schema and then loads the initializer file.


  The initializer file is a Ruby file, so you can define associations within it, for example:

  #+begin_src ruby
    module Blog
      class Student
        has_many :courses, foreign_key: :student_id, class_name: 'Course'
        belongs_to :school, foreign_key: :school_id, class_name: 'School'

        has_and_belongs_to_many :teachers, join_table: 'students_teachers', foreign_key: :student_id, association_foreign_key: :teacher_id, class_name: 'Teacher'
      end

      class Course
        belongs_to :student, foreign_key: :student_id, class_name: 'Student'
      end

      class School
        has_many :students, foreign_key: :school_id, class_name: 'Student'
      end

      class Teacher
        has_and_belongs_to_many :students, join_table: 'students_teachers', foreign_key: :teacher_id, association_foreign_key: :student_id, class_name: 'Student'
      end
    end
  #+end_src

  1.  =has_one= Indicates that this table is a one-to-one relationship with the owner
  2. =belongs_to= Indicates that this table is a subordinate of a one-to-many or one-to-one relationship
  3. =has_and_belongs_to_many= Indicates that this table is one of the parties to a many-to-many relationship
  4. =class_name= The model class name of the other party that indicates the relationship (the model class name is
     actually the CamelCase form of the table name)
  5. =foreign_key= Indicates the name of the associated field on the side of the dependent table in the association
  6. =primary_key= Indicates the name of the association field on the side of the master table in the association
  7. =join_table= In a many-to-many relationship, the name of the intermediate table that is associated with two tables is
     indicated
  8. =association_foreign_key= In a many-to-many relationship, indicates the name of the field associated with the other
     model's model in the intermediate table

  可以参考： [[https://guides.rubyonrails.org/association_basics.html]]


  Considering that the model classes are all defined under the Namespace module, a blog here is necessary.


  Of course, Arql will load the =~/.arql.rb= OR =~/.arql.d/init.rb= file by default, regardless of which environment is
  selected via the =-e= options, so putting a fixed namespace =Blog= in the default initialization file like in the
  example above is not a good choice.

  There are two ways to solve this problem:

  1.
     When using arql, for different environments, use =-i= the option to specify different initialization files, such as:
     =arql -e blog -i ~/.arql.d/blog.rb=
  2. Refer to Put the initialization code for different environments in different files