Vapor4.0数据库增加字段

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Question is following: How to implement additional migration on already existing table without revert or recreating DB?
Original migration:

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struct CreateTile: Migration {
    // Prepares the database for storing Tile models.
    func prepare(on database: Database) -> EventLoopFuture<Void> {
        database.schema("tiles")
            .id()
            .field("x", .int, .required)
            .field("y", .int, .required)
            .field("z", .int, .required)
            .create()
    }

    // Optionally reverts the changes made in the prepare method.
    func revert(on database: Database) -> EventLoopFuture<Void> {
        database.schema("tiles").delete()
    }
}

Now you want to extend the table by additional column called new which is of type Int. Therefore you have to implement new migration.

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struct UpdateTile: Migration {
    // Prepares the database for storing Tile models.
    func prepare(on database: Database) -> EventLoopFuture<Void> {
        database.schema("tiles")
            .field("new", .int)
            .update()
    }

    // Optionally reverts the changes made in the prepare method.
    func revert(on database: Database) -> EventLoopFuture<Void> {
        database.schema("tiles").deleteField("new").update()
    }
}

Last step is to register your new migration in configure.swift file similar as the original one