Make progress on related objects.

Frameworks/RSDatabase/Related Objects/DatabaseRelatedObjectsTable.swift

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+//
+//  DatabaseRelatedObjectsTable.swift
+//  RSDatabase
+//
+//  Created by Brent Simmons on 9/2/17.
+//  Copyright © 2017 Ranchero Software, LLC. All rights reserved.
+//
+
+import Foundation
+
+public protocol DatabaseRelatedObjectsTable: DatabaseTable {
+
+	var databaseIDKey: String { get}
+
+	func fetchObjectsWithIDs(_ databaseIDs: Set<String>, in database: FMDatabase) -> [DatabaseObject]
+	func objectsWithResultSet(_ resultSet: FMResultSet) -> [DatabaseObject]
+	func objectWithRow(_ row: FMResultSet) -> DatabaseObject?
+
+	func save(_ objects: [DatabaseObject], in database: FMDatabase)
+}
+
+public extension DatabaseRelatedObjectsTable {
+
+	// MARK: Default implementations
+
+	func fetchObjectsWithIDs(_ databaseIDs: Set<String>, in database: FMDatabase) -> [DatabaseObject] {
+
+		guard let resultSet = selectRowsWhere(key: databaseIDKey, inValues: Array(databaseIDs), in: database) else {
+			return [DatabaseObject]()
+		}
+		return objectsWithResultSet(resultSet)
+	}
+
+	func objectsWithResultSet(_ resultSet: FMResultSet) -> [DatabaseObject] {
+
+		return resultSet.flatMap(objectWithRow)
+	}
+}