I’m in chapter 23 where you migrate the category names to being unique. It happens that I had two categories im my db that don’t have unique names. For obvious reasons you can’t migrate the database to unique names in that case. You first have to migrate the data and move all acronyms of a duplicate category to the canonical category.
Unfortunately the book doesn’t even seem to mention the topic of data migrations.
It might be a useful addition to show how to deal with data migrations. One would of course also need some unittests for that. To be honest, it sounds like a whole new chapter in the book.
So that’s probably something to be saved for a new edition, or even an “advanced Vapor” book.
After playing around a while, I came up with this. An experienced vapor user could probably improve that tremendously. I don’t really grasp the concept of Futures fully. Nevertheless, it works:
struct MakeCategoriesUnique: Migration {
typealias Database = PostgreSQLDatabase
static func prepare(on conn: PostgreSQLConnection) -> EventLoopFuture<Void> {
return Category.query(on: conn).all().flatMap(to: Void.self) { categories in
if categories.count == Set(categories.map { $0.name }).count {
print("No duplicate categories")
// no duplicate categories
return conn.future()
}
let duplicates = Dictionary(grouping: categories, by: { $0.name }).filter { $1.count > 1 }
var categoryOperations: [Future<Void>] = []
for (_, categories) in duplicates {
let uniqueCategory = categories.first!
print("Uniquing category \"\(uniqueCategory.name) [ID: \(try uniqueCategory.requireID())\"")
// get all acronyms for that category
for category in categories[1...] {
try categoryOperations.append(category.acronyms.query(on: conn).all().flatMap(to: Void.self) { acronyms in
var acronymUpdates: [Future<Void>] = []
for acronym in acronyms {
print("Unique category of acronym \"\(acronym.short)\"")
acronymUpdates.append(flatMap(to: Void.self,
acronym.categories.detach(category, on: conn),
acronym.categories.isAttached(uniqueCategory, on: conn).map { hasUniqueCategory in
if !hasUniqueCategory {
print("Assign unique category to acronym \"\(acronym.short)\"")
acronymUpdates.append(acronym.categories.attach(uniqueCategory, on: conn).transform(to: ()))
}
else {
print("Acronym \"\(acronym.short)\" already has unique category")
}
},
acronym.save(on: conn)) { _, _, _ in
return conn.future()
})
}
return acronymUpdates.flatten(on: conn)
})
categoryOperations.append(category.delete(on: conn).transform(to: ()))
}
}
return categoryOperations.flatten(on: conn)
}.flatMap(to: Void.self) { future in
Database.update(Category.self, on: conn) { builder in
builder.unique(on: \.name)
}
}
}
static func revert(on conn: PostgreSQLConnection) -> EventLoopFuture<Void> {
return Database.update(Category.self, on: conn) { builder in
// uniquing data is destructive, so we can't undo the above data migration
builder.deleteUnique(from: \.name)
}
}
}
