implement proper PSQL upsert support

Author: tanner0101

Sources/FluentPostgreSQL/Fluent+PostgreSQLUpsert.swift

@@ -0,0 +1,19 @@
+extension _PostgreSQLModel {
+    public func create(orUpdate: Bool, on conn: DatabaseConnectable) -> Future<Self> {
+        return Self.query(on: conn).create(orUpdate: orUpdate, self)
+    }
+}
+
+extension QueryBuilder where Result: _PostgreSQLModel, Result.Database == Database {
+    public func create(orUpdate: Bool, _ model: Result) -> Future<Result> {
+        if orUpdate {
+            let row = SQLQueryEncoder(PostgreSQLExpression.self).encode(model)
+            let values = row.map { row -> (PostgreSQLIdentifier, PostgreSQLExpression) in
+                return (.identifier(row.key), row.value)
+            }
+            self.query.upsert = .upsert([.keyPath(Result.idKey)], values)
+        }
+        return create(model)
+    }
+    
+}

Sources/FluentPostgreSQL/PostgreSQLModel.swift

@@ -1,4 +1,13 @@
-public protocol PostgreSQLModel: Model where Self.Database == PostgreSQLDatabase, Self.ID == Int {
+public protocol _PostgreSQLModel: Model, PostgreSQLTable where Self.Database == PostgreSQLDatabase { }
+
+extension _PostgreSQLModel {
+    /// See `SQLTable`.
+    public static var sqlTableIdentifierString: String {
+        return entity
+    }
+}
+
+public protocol PostgreSQLModel: _PostgreSQLModel where Self.ID == Int {
     /// This model's unique identifier.
     var id: Int? { get set }
 }

Sources/FluentPostgreSQL/PostgreSQLStringModel.swift

@@ -1,6 +1,4 @@
-import Foundation
-
-public protocol PostgreSQLStringModel: Model where Self.Database == PostgreSQLDatabase, Self.ID == String {
+public protocol PostgreSQLStringModel: _PostgreSQLModel where Self.ID == String {
     /// This model's unique identifier.
     var id: String? { get set }
 }

Sources/FluentPostgreSQL/PostgreSQLUUIDModel.swift

@@ -1,4 +1,4 @@
-public protocol PostgreSQLUUIDModel: Model where Self.Database == PostgreSQLDatabase, Self.ID == UUID {
+public protocol PostgreSQLUUIDModel: _PostgreSQLModel where Self.ID == UUID {
     /// This model's unique identifier.
     var id: UUID? { get set }
 }