Remove Foreign Key support

Author: aaronpearce

Harmony/Core/Harmonic.swift

@@ -275,48 +275,25 @@ private extension Harmonic {
     func handleFetchedRecordZoneChanges(_ event: CKSyncEngine.Event.FetchedRecordZoneChanges) {
         Logger.database.info("Handle fetched record zone changes \(event)")
 
-        var deferredRecords = [any HRecord]()
         for modification in event.modifications {
             // The sync engine fetched a record, and we want to merge it into our local persistence.
             // If we already have this object locally, let's merge the data from the server.
             // Otherwise, let's create a new local object.
             let record = modification.record
             if let id = record.recordID.parsedRecordID,
                let modelType = modelType(for: record) {
-                try! database.writeWithDeferredForeignKeys { db in
+                try! database.write { db in
                     if var localRecord = try modelType.fetchOne(db, key: UUID(uuidString: id)) {
                         try localRecord.updateChanges(db: db, ckRecord: record)
                     } else {
                         if let model = modelType.parseFrom(record: record) {
-                            // By catching `SQLITE_CONSTRAINT_FOREIGNKEY` it allows us to defer this records to the end of this batch.
-                            // This will fail to handle if the parent record isn't in the same batch. Maybe a instance-wide "deferred" array to check each time.
-                            // Alternatively, we could use this method and just YOLO it into the database anyway.
-                            // https://github.com/groue/GRDB.swift/issues/172
-                            do {
-                                try model.save(db)
-                            } catch ResultCode.SQLITE_CONSTRAINT_FOREIGNKEY {
-                                // Defer and try again later
-                                deferredRecords.append(model)
-                            } catch {
-                                throw error
-                            }
+                            try model.save(db)
                         }
                     }
                 }
             }
         }
 
-        // Write deferred records with no foreign key checks to avoid conflicts for now.
-        do {
-            try database.writeWithDeferredForeignKeys { db in
-                for deferredRecord in deferredRecords {
-                    try deferredRecord.save(db)
-                }
-            }
-        } catch {
-            print(error.localizedDescription)
-        }
-
         for deletion in event.deletions {
 
             // A record was deleted on the server, so let's remove it from our local persistence.
@@ -348,7 +325,7 @@ private extension Harmonic {
         for savedRecord in event.savedRecords {
             if let id = savedRecord.recordID.parsedRecordID,
                let modelType = modelType(for: savedRecord) {
-                try! database.writeWithDeferredForeignKeys { db in
+                try! database.write { db in
                     var localRecord = try? modelType.fetchOne(db, key: UUID(uuidString: id))
                     localRecord?.setLastKnownRecordIfNewer(savedRecord)
                     try! localRecord?.save(db)
@@ -375,7 +352,7 @@ private extension Harmonic {
                     continue
                 }
 
-                try? database.writeWithDeferredForeignKeys { db in
+                try? database.write { db in
                     var localRecord = try modelType.fetchOne(db, key: UUID(uuidString: id))
                     // Merge from server...
                     try localRecord?.updateChanges(db: db, ckRecord: serverRecord)
@@ -411,7 +388,7 @@ private extension Harmonic {
             }
 
             if shouldClearServerRecord {
-                try? database.writeWithDeferredForeignKeys { db in
+                try? database.write { db in
                     var localRecord = try? modelType.fetchOne(db, key: UUID(uuidString: id))
                     // Merge from server...
                     localRecord?.archivedRecord = nil

Harmony/Extensions/DatabaseWriter+WriteDeferred.swift

@@ -29,3 +29,7 @@ You can then access your Harmony instance from anywhere for writing or reading b
 Harmony provides access to write methods that will automatically sync to CloudKit and hides direct database writing access for this purpose. If you write via another means to your GRDB database, Harmony will not see those changes.
 
 Harmony also provides direct access to your `DatabaseReader` if you wish to read directly. I highly suggest seeing if GRDBQuery will fit your needs first. 
+
+### Foreign Keys
+
+Harmony does not support usage of foreign keys in GRDB. This is due to CloudKit being a system of eventual consistency. Records can return in sets where they don't have the related data within the same set or out of order. Foreign keys are incompatible with this style of system so we have taken the choice to not support foreign keys after many attempts to do so.