HasManyThroughAssociation

For example, consider the practice of passport delivery. One country “has many” citizens “through” its passports:

struct Citizen: TableRecord { }

struct Passport: TableRecord {
    static let citizen = belongsTo(Citizen.self)
}

struct Country: TableRecord {
    static let passports = hasMany(Passport.self)
    static let citizens = hasMany(Citizen.self,
                                  through: passports,
                                  using: Passport.citizen)
}

The HasManyThroughAssociation is also useful for setting up “shortcuts” through nested associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way:

struct Paragraph: TableRecord { }

struct Section: TableRecord {
    static let paragraphs = hasMany(Paragraph.self)
}

struct Document: TableRecord {
    static let sections = hasMany(Section.self)
    static let paragraphs = hasMany(Paragraph.self,
                                    through: sections,
                                    using: Section.paragraphs)
}

As in the examples above, HasManyThroughAssociation is always built from two other associations. Those associations can be any Association.