model has_many users(of a specific role type)

Question

I have a pretty standard devise user model schema with roles and a many to many model for userRoles:

  #teacher or student
  create_table "roles", :force => true do |t|
    t.string   "name"
  end

  create_table "user_roles", :force => true do |t|
    t.integer  "user_id"
    t.integer  "role_id"
  end    
  add_index "user_roles", ["role_id"], :name => "index_user_roles_on_role_id"
  add_index "user_roles", ["user_id"], :name => "index_user_roles_on_user_id"

  create_table "users", :force => true do |t|
    t.string   "email",                  :default => "", :null => false
    t.string   "encrypted_password",     :default => "", :null => false
    ........more stuff....
  end    
  add_index "users", ["email"], :name => "index_users_on_email", :unique => true
  add_index "users", ["reset_password_token"], :name => "index_users_on_reset_password_token", :unique => true

end

The roles are teacher and student. I want to create a Clazz (as in school class) model with many to many students (users with role of students) and many to many teachers (users with role of teachers).

I think it would look something like this, but can't quite get it:

rails g scaffold Clazz name:string time:datetime has_many:users(of type student) has_many:users(of type teacher) How do I do the last two? Do I create the has_many, :through association migrations (teacherClass and studentClass) first? or is there a helper method?

Maybe I just couldn't find an appropriate tutorial or example, so i could use one of those as well... Thanks in advance

---

---

This is more information about my attempt at @Cody Caughlan 's answer

Schema.rb :

ActiveRecord::Schema.define(:version => 20130523111519) do

  create_table "class_instructions", :force => true do |t|
    t.string   "name"
    t.datetime "time"
    t.integer  "user_id"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  add_index "class_instructions", ["user_id"], :name => "index_class_instructions_on_user_id"

  create_table "roles", :force => true do |t|
    t.string   "name"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  create_table "songs", :force => true do |t|
    t.string   "title"
    t.string   "content"
    t.datetime "created_at",             :null => false
    t.datetime "updated_at",             :null => false
    t.integer  "user_id"
    t.string   "fractionRepresentation"
    t.string   "measureRepresentation"
  end

  create_table "user_roles", :force => true do |t|
    t.integer  "user_id"
    t.integer  "role_id"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  add_index "user_roles", ["role_id"], :name => "index_user_roles_on_role_id"
  add_index "user_roles", ["user_id"], :name => "index_user_roles_on_user_id"

  create_table "users", :force => true do |t|
    t.string   "email",                  :default => "", :null => false
    t.string   "encrypted_password",     :default => "", :null => false
    t.string   "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.integer  "sign_in_count",          :default => 0
    t.datetime "current_sign_in_at"
    t.datetime "last_sign_in_at"
    t.string   "current_sign_in_ip"
    t.string   "last_sign_in_ip"
  end

  add_index "users", ["email"], :name => "index_users_on_email", :unique => true
  add_index "users", ["reset_password_token"], :name => "index_users_on_reset_password_token", :unique => true

end

User.rb model :

class User < ActiveRecord::Base
  #DEVISE STUFF

  #== Associations
  has_many :user_roles, :dependent => :destroy
  has_many :roles, :through => :user_roles

  has_many :songs    

  #== Instance Methods
  def thisUsersID
    self.id
  end

  def student_classes(class_instruction_name)
    member_classes(class_instruction_name, 'Student')
  end

  def teacher_classes(class_instruction_name)
    member_classes(class_instruction_name, Role::TEACHER)
  end

  private

  def member_classes(class_instruction_name, type)
   ClassInstruction \
     .joins(:user_role) \
     .where(["user_role.user_id = ?", id]) \
     .joins("INNER JOIN roles ON roles.id = user_roles.role_id") \
     .where("roles.name = ?", type) \
     .where("class_instruction.name = ?", class_instruction_name)
  end
end

user_role.rb model:

class UserRole < ActiveRecord::Base
  belongs_to :user
  belongs_to :role
  has_many :class_instructions, :dependent => :destroy
end

role.rb model

class Role < ActiveRecord::Base
  has_many :user_roles, :dependent => :destroy
end

class_instruction.rb model (names are 'ss' and'math-101')

class ClassInstruction < ActiveRecord::Base
  belongs_to :user_role
end

Error Stack

Loading development environment (Rails 3.2.2)

1.9.3-p429 :001 > U = User.find(3)

  User Load (3.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1  [["id", 3]]
 => #<User id: 3, email: "stu@test.org", encrypted_password: "$2a$10$2DxWWV34BRFJoLboYyWCIeXEtCPYOSe8JqpTmFU6W2i1...", reset_password_token: nil, reset_password_sent_at: nil, remember_created_at: nil, sign_in_count: 0, current_sign_in_at: nil, last_sign_in_at: nil, current_sign_in_ip: nil, last_sign_in_ip: nil> 

1.9.3-p429 :002 > U.student_classes('math-101').all

  ClassInstruction Load (0.2ms)  SELECT "class_instructions".* FROM "class_instructions" INNER JOIN "user_roles" ON "user_roles"."id" = "class_instructions"."user_role_id" INNER JOIN roles ON roles.id = user_roles.role_id WHERE (user_role.user_id = 3) AND (roles.name = 'Student') AND (class_instruction.name = 'math-101')
SQLite3::SQLException: no such column: user_role.user_id: SELECT "class_instructions".* FROM "class_instructions" INNER JOIN "user_roles" ON "user_roles"."id" = "class_instructions"."user_role_id" INNER JOIN roles ON roles.id = user_roles.role_id WHERE (user_role.user_id = 3) AND (roles.name = 'Student') AND (class_instruction.name = 'math-101')
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: user_role.user_id: SELECT "class_instructions".* FROM "class_instructions" INNER JOIN "user_roles" ON "user_roles"."id" = "class_instructions"."user_role_id" INNER JOIN roles ON roles.id = user_roles.role_id WHERE (user_role.user_id = 3) AND (roles.name = 'Student') AND (class_instruction.name = 'math-101')
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/sqlite3-1.3.7/lib/sqlite3/database.rb:91:in `initialize'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/sqlite3-1.3.7/lib/sqlite3/database.rb:91:in `new'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/sqlite3-1.3.7/lib/sqlite3/database.rb:91:in `prepare'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/connection_adapters/sqlite_adapter.rb:246:in `block in exec_query'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/connection_adapters/abstract_adapter.rb:280:in `block in log'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activesupport-3.2.2/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/connection_adapters/abstract_adapter.rb:275:in `log'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/connection_adapters/sqlite_adapter.rb:242:in `exec_query'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/connection_adapters/sqlite_adapter.rb:460:in `select'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/connection_adapters/abstract/database_statements.rb:18:in `select_all'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/connection_adapters/abstract/query_cache.rb:63:in `select_all'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/querying.rb:38:in `block in find_by_sql'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/explain.rb:40:in `logging_query_plan'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/querying.rb:37:in `find_by_sql'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/relation.rb:171:in `exec_queries'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/relation.rb:160:in `block in to_a'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/explain.rb:33:in `logging_query_plan'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/relation.rb:159:in `to_a'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/activerecord-3.2.2/lib/active_record/relation/finder_methods.rb:159:in `all'
    from (irb):2
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/railties-3.2.2/lib/rails/commands/console.rb:47:in `start'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/railties-3.2.2/lib/rails/commands/console.rb:8:in `start'
    from /Users/phycom06/.rvm/gems/ruby-1.9.3-p429/gems/railties-3.2.2/lib/rails/commands.rb:41:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'1.9.3-p429 :003 > 

Answer 1

Firstly, Class is a reserved word in Ruby so you will not be able to use it as a Model name.

http://www.java2s.com/Code/Ruby/Language-Basics/Rubysreservedwords.htm

So pick a different name. In cases like this I have done Clazz.

As for the data model - yes you would specify the relationships in the ActiveRecord class definition and not in the migration.

Its not clear what your Clazz model looks like. But I will assume it has 2 columns: name and a user_role_id. You'd probably want to pull out the name into a separate table and have it be replaced with like a clazz_id - but for now we will keep it flat.

I will use an instance method to build the associations, but you might be able to get crazy in the has_many definitions.

class Clazz < ActiveRecord::Base
  belongs_to :user_role
end

class User

 #== Associations
 has_many :user_roles, :dependent => :destroy
 has_many :roles, :through => :user_roles

 #== Instance Methods
 def student_classes(clazz_name)
  member_classes(clazz_name, Role::STUDENT)
 end

 def teacher_classes(clazz_name)
  member_classes(clazz_name, Role::TEACHER)
 end

 private

 def member_classes(clazz_name, type)
   Clazz \
     .joins(:user_role) \
     .where(["user_role.user_id = ?", id]) \
     .joins("INNER JOIN roles ON roles.id = user_roles.role_id") \
     .where("roles.name = ?", type) \
     .where("clazzes.name = ?", clazz_name)
 end

end

class UserRole
 has_many :clazzes, :dependent => :destroy
end

class Role
  has_many :user_roles, :dependent => :destroy
end

So this will enable you to do something like:

user = User.find(99) # we know this user is a student
user.student_classes('Social Studies 101').all

Something like that.