Support for single value decoding

GRDB/Record/FetchableRecord+Decodable.swift

@@ -108,8 +108,12 @@ private struct _RowDecoder<R: FetchableRecord>: Decoder {
 
     func singleValueContainer() throws -> SingleValueDecodingContainer {
         guard let key = codingPath.last else {
-            // Decoding an array of scalars from rows: pick the first column
-            return ColumnDecoder<R>(row: row, columnIndex: 0, codingPath: codingPath)
+            // Not yet sure what we are decoding, this will be decided in the SingleValueDecodingContainer functions.
+            // For decoding an array of scalars (in case of prefetched rows) we pick the first column.
+            return SingleValueRowDecoder(
+                columnDecoder: ColumnDecoder<R>(row: row, columnIndex: 0, codingPath: codingPath),
+                columnDecodingStrategy: columnDecodingStrategy
+            )
         }
         guard let index = row.index(forColumn: key.stringValue) else {
             // Don't use DecodingError.keyNotFound:
@@ -346,7 +350,7 @@ private struct _RowDecoder<R: FetchableRecord>: Decoder {
 
             // Unknown key
             //
-            // Should be throw an error? Well... The use case is the following:
+            // Should we throw an error? Well... The use case is the following:
             //
             //      // SELECT book.*, author.* FROM book
             //      // JOIN author ON author.id = book.authorId
@@ -487,6 +491,50 @@ private struct _RowDecoder<R: FetchableRecord>: Decoder {
     }
 }
 
+private struct SingleValueRowDecoder<R: FetchableRecord>: SingleValueDecodingContainer {
+    var columnDecoder: ColumnDecoder<R>
+    var columnDecodingStrategy: DatabaseColumnDecodingStrategy
+    let codingPath: [any CodingKey] = []
+
+    func decodeNil() -> Bool { columnDecoder.decodeNil() }
+    func decode(_ type: Bool.Type) throws -> Bool { try columnDecoder.decode(type) }
+    func decode(_ type: String.Type) throws -> String { try columnDecoder.decode(type) }
+    func decode(_ type: Double.Type) throws -> Double { try columnDecoder.decode(type) }
+    func decode(_ type: Float.Type) throws -> Float { try columnDecoder.decode(type) }
+    func decode(_ type: Int.Type) throws -> Int { try columnDecoder.decode(type) }
+    func decode(_ type: Int8.Type) throws -> Int8 { try columnDecoder.decode(type) }
+    func decode(_ type: Int16.Type) throws -> Int16 { try columnDecoder.decode(type) }
+    func decode(_ type: Int32.Type) throws -> Int32 { try columnDecoder.decode(type) }
+    func decode(_ type: Int64.Type) throws -> Int64 { try columnDecoder.decode(type) }
+#if compiler(>=6)
+    @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
+    func decode(_ type: Int128.Type) throws -> Int128 { try columnDecoder.decode(type) }
+#endif
+    func decode(_ type: UInt.Type) throws -> UInt { try columnDecoder.decode(type) }
+    func decode(_ type: UInt8.Type) throws -> UInt8 { try columnDecoder.decode(type) }
+    func decode(_ type: UInt16.Type) throws -> UInt16 { try columnDecoder.decode(type) }
+    func decode(_ type: UInt32.Type) throws -> UInt32 { try columnDecoder.decode(type) }
+    func decode(_ type: UInt64.Type) throws -> UInt64 { try columnDecoder.decode(type) }
+#if compiler(>=6)
+    @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
+    func decode(_ type: UInt128.Type) throws -> UInt128 { try columnDecoder.decode(type) }
+#endif
+
+    func decode<T>(_ type: T.Type) throws -> T where T: Decodable {
+        if let type = T.self as? any FetchableRecord.Type {
+            // Prefer FetchableRecord decoding over Decodable.
+            return try type.init(row: columnDecoder.row) as! T
+        } else {
+            let decoder = _RowDecoder<R>(
+                row: columnDecoder.row,
+                codingPath: [],
+                columnDecodingStrategy: columnDecodingStrategy
+            )
+            return try T(from: decoder)
+        }
+    }
+}
+
 // MARK: - PrefetchedRowsDecoder
 
 private struct PrefetchedRowsDecoder<R: FetchableRecord>: Decoder {

Tests/GRDBTests/FetchableRecordDecodableTests.swift

@@ -1913,4 +1913,70 @@ extension FetchableRecordDecodableTests {
             }
         }
     }
+
+    // Regression test for <https://github.com/groue/GRDB.swift/issues/1572>
+    func testSingleValueContainer() throws {
+        struct Struct: Decodable {
+            let value: String
+        }
+
+        struct Wrapper<Model: Decodable>: FetchableRecord, Decodable {
+            var model: Model
+            var otherValue: String
+
+            enum CodingKeys: String, CodingKey {
+                case otherValue
+            }
+
+            init(from decoder: any Decoder) throws {
+                let container = try decoder.container(keyedBy: CodingKeys.self)
+                otherValue = try container.decode(String.self, forKey: . otherValue)
+
+                let singleValueContainer = try decoder.singleValueContainer()
+                model = try singleValueContainer.decode(Model.self)
+            }
+        }
+
+        let row = Row(["value": "foo", "otherValue": "bar"])
+
+        let wrapper = try Wrapper<Struct>(row: row)
+        XCTAssertEqual(wrapper.model.value, "foo")
+        XCTAssertEqual(wrapper.otherValue, "bar")
+    }
+
+    // Regression test for <https://github.com/groue/GRDB.swift/issues/1572>
+    // Here we test that `FetchableRecord` takes precedence over `Decodable`
+    // when a record is encoded with a `SingleValueEncodingContainer`.
+    func testSingleValueContainerWithFetchableRecord() throws {
+        struct Struct: Decodable, FetchableRecord {
+            let value: String
+
+            init(row: Row) throws {
+                value = row["actualValue"]
+            }
+        }
+
+        struct Wrapper<Model: Decodable>: FetchableRecord, Decodable {
+            var model: Model
+            var otherValue: String
+
+            enum CodingKeys: String, CodingKey {
+                case otherValue
+            }
+
+            init(from decoder: any Decoder) throws {
+                let container = try decoder.container(keyedBy: CodingKeys.self)
+                otherValue = try container.decode(String.self, forKey: . otherValue)
+
+                let singleValueContainer = try decoder.singleValueContainer()
+                model = try singleValueContainer.decode(Model.self)
+            }
+        }
+
+        let row = Row(["value": "foo", "otherValue": "bar", "actualValue": "test"])
+
+        let wrapper = try Wrapper<Struct>(row: row)
+        XCTAssertEqual(wrapper.model.value, "test")
+        XCTAssertEqual(wrapper.otherValue, "bar")
+    }
 }