Define a unique primary key based on 2 columns

Question

I would like to define a unique key for records based on 2 columns : \'id\' and \'language\'

to let the user submits the following strings : id=1 language=en value=bl

Answer 1

In Rails 5 you can do the following:

create_table :words, primary_key: %i[id language_id] do |t|
  t.integer :id
  t.integer :language_id
  t.string :value
  t.timestamps
end

It is also NOT necessary to set the primary_key attribute on the Word model.

Answer 2

add_index :words, ["id", "language_id"], :unique => true

It should work. Maybe you have already some non-unique data in your db and index can't be created? But (as @Doon noticed it will be redundant since id is always unique). So you need create primary key on two columns.

To define 2 column primary key in rails use:

create_table :words, {:id => false} do |t|
  t.integer :id
  t.integer :language_id
  t.string :value
  t.timestamps
end
execute "ALTER TABLE words ADD PRIMARY KEY (id,language_id);"

And set primary_key in your model with this gem: http://rubygems.org/gems/composite_primary_keys:

class Word < ActiveRecord::Base
    self.primary_keys = :id,:language_id
end

Answer 3

Just like @rogal111 said, but if a primary key already exists then you'll want to do this

ALTER TABLE sections DROP PRIMARY KEY, ADD PRIMARY KEY(id, workspace_id, section_key);

Answer 4

As I said in my comments you will be fighting rails if you try this, and it isn't really supported out of the box. you can look at http://compositekeys.rubyforge.org which offers a way to do composite primary keys in rails. I haven't used it, as I haven't had a need yet (normally when I have something that is composite key like it is just a join table with no primary key and a unique index on the joined pair (HABTM).

Answer 5

I faced a similar problem when migrating a site to Rails. I had a table which stores text data for each language my site is available in so I had something like this:

CREATE TABLE Project_Lang(
    project_id INT NOT NULL,
    language_id INT NOT NULL,
    title VARCHAR(80),
    description TEXT,

    PRIMARY KEY pk_Project_Lang(project_id, language_id),

    FOREIGN KEY fk_Project_Lang_Project(project_id)
        REFERENCES Project(project_id)
        ON DELETE RESTRICT ON UPDATE CASCADE,

    FOREIGN KEY fk_Project_Lang_Language(language_id)
        REFERENCES Language(language_id)
        ON DELETE RESTRICT ON UPDATE CASCADE
)ENGINE = InnoDB DEFAULT CHARACTER SET = utf8 DEFAULT COLLATE = utf8_spanish_ci;

But since Rails doesn't handle composite primary keys out of the box I was forced to change the structure of the table so it had it's own primary key:

CREATE TABLE Project_Lang(
    project_lang_id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
    project_id INT NOT NULL,
    language_id INT NOT NULL,
    title VARCHAR(80),
    description TEXT,

    UNIQUE INDEX(project_id, language_id),

    FOREIGN KEY fk_Project_Lang_Project(project_id)
        REFERENCES Project(project_id)
        ON DELETE RESTRICT ON UPDATE CASCADE,

    FOREIGN KEY fk_Project_Lang_Language(language_id)
        REFERENCES Language(language_id)
        ON DELETE RESTRICT ON UPDATE CASCADE
)ENGINE = InnoDB DEFAULT CHARACTER SET = utf8 DEFAULT COLLATE = utf8_spanish_ci;

I also created a unique index for the columns that previously made the composite primary key so that no duplicate record is inserted. Then in my Rails model I could simply:

self.primary_key = "project_lang_id"

And that did the trick. Is not what I wanted but is better than fighting the framework.

Answer 6

Model

class User < ActiveRecord::Base
  has_secure_password
  self.primary_keys = :name
end

Migration

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name, null: false
      t.string :emailid
      t.string :password_digest
      t.integer :locked, :default => 0
      t.text :secretquestion
      t.string :answer

      t.timestamps null: false
    end
    add_index :users, :name, :unique => true
  end
end

You will get this table