ForeignKey

A ForeignKey helps building associations when GRDB can’t infer a foreign key from the database schema.

Sometimes the database schema does not define any foreign key between two tables. And sometimes, there are several foreign keys from a table to another:

| Table book   |       | Table person |
| ------------ |       | ------------ |
| id           |   +-->• id           |
| authorId     •---+   | name         |
| translatorId •---+
| title        |

When this happens, associations can’t be automatically inferred from the database schema. GRDB will complain with a fatal error such as “Ambiguous foreign key from book to person”, or “Could not infer foreign key from book to person”.

Your help is needed. You have to instruct GRDB which foreign key to use:

struct Book: TableRecord {
    // Define foreign keys
    static let authorForeignKey = ForeignKey(["authorId"]))
    static let translatorForeignKey = ForeignKey(["translatorId"]))

    // Use foreign keys to define associations:
    static let author = belongsTo(Person.self, using: authorForeignKey)
    static let translator = belongsTo(Person.self, using: translatorForeignKey)
}

Foreign keys are always defined from the table that contains the columns at the origin of the foreign key. Person’s symmetric HasMany associations reuse Book’s foreign keys:

struct Person: TableRecord {
    static let writtenBooks = hasMany(Book.self, using: Book.authorForeignKey)
    static let translatedBooks = hasMany(Book.self, using: Book.translatorForeignKey)
}

Foreign keys can also be defined from query interface columns:

struct Book: TableRecord {
    enum Columns: String, ColumnExpression {
        case id, title, authorId, translatorId
    }

    static let authorForeignKey = ForeignKey([Columns.authorId]))
    static let translatorForeignKey = ForeignKey([Columns.translatorId]))
}

When the destination table of a foreign key does not define any primary key, you need to provide the full definition of a foreign key:

struct Book: TableRecord {
    static let authorForeignKey = ForeignKey(["authorId"], to: ["id"]))
    static let author = belongsTo(Person.self, using: authorForeignKey)
}