Adding Foreign Keys to a Database in Vapor

Foreign key constraints describe a link between two tables.

Example:

final class Technique: Codable {
  var id : Int?
  let name : String
  var description: String
  var heatID: Heat.ID
  
  init(name: String, description: String, heatID: Heat.ID) {
    self.name = name
    self.description = description
    self.heatID = heatID
  }
}

Here the Technique model needs a Heat ID.

To add referential integrity, we need to conform to Migrations protocol:

extension Technique : Migration {
  
  static func prepare(on connection: PostgreSQLConnection) -> Future<Void> {
    return Database.create(self, on: connection) { builder in
      
      try addProperties(to: builder)
      
      // Add FOREIGN KEY reference, with ON UPDATE/DELETE CASCADE actions
      try builder.addReference(from: \.heatID, to: \Heat.id, actions: .update)
      
      // make name UNIQUE and indexed
      try builder.addIndex(to: \.name, isUnique: true)
    }
  }
}

In the above example the relationship is one - to - many between Heat and Technique.

Heat ↤⇉ Technique

And many - to - one between Technique and Heat.

Technique ⇇↦ Heat

Prev: Adding Indexes and Constraints to a Database in Vapor

Next: Pre-populating a Database with Data in Vapor

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