Creating Common Database Relationships - Gaelan/mrhs-blog GitHub Wiki

Many To Many

For associations where both sides are related to many of the other. For example, Courses and Units where each course will have many units and each unit may be used in more than one course.

Generate the scaffold or model (scaffold objects near the top of the hierarchy, generate models for included objects):

Do this for each model that you are creating, some of the models may already exist if you are making changes to an existing application. Skip this step if both models already exist.
```
bin/rails g scaffold Unit title:string soi:text duration:integer
```
If you are generating a scaffold, you may be prompted to overwrite the file scaffolds.scss:
```
    create      app/assets/javascripts/units.coffee
    invoke    scss
    create      app/assets/stylesheets/units.scss
    invoke  scss
  conflict    app/assets/stylesheets/scaffolds.scss
Overwrite /Users/dlu/Source/Ruby/Rails/mrhs-blog/app/assets/stylesheets/scaffolds.scss? (enter "h" for help) [Ynaqdh]
```
Respond n (don't overwrite).

Generate the association model

bin/rails g model CourseUnit course:belongs_to unit:belongs_to

Add the associations to the models

In app/models/course.rb:

has_many :course_units
has_many :units, through: :course_units

In app/models/unit.rb:

has_many :course_units
has_many :courses, through: :course_units

Run the migrations
```
bin/rails db:migrate
```