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

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.