Improve query decoding errors

Let's surface more information about which column failed to decode.

Author: stephencelis

SharingGRDB.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -25,6 +25,29 @@ public class QueryCursor<Element>: DatabaseCursor {
   public func _element(sqliteStatement _: SQLiteStatement) throws -> Element {
     fatalError("Abstract method should be overridden in subclass")
   }
+
+  @usableFromInline
+  struct DecodingError: Error, CustomStringConvertible {
+    let columnIndex: Int
+    let columnName: String
+    let sql: String
+
+    @usableFromInline
+    init(columnIndex: Int, columnName: String, sql: String) {
+      self.columnIndex = columnIndex
+      self.columnName = columnName
+      self.sql = sql
+    }
+
+    @usableFromInline
+    var description: String {
+      """
+      Expected column \(columnIndex) (\(columnName.debugDescription)) to not be NULL: …
+
+      \(sql)
+      """
+    }
+  }
 }
 
 @usableFromInline
@@ -40,9 +63,18 @@ final class QueryValueCursor<QueryValue: QueryRepresentable>: QueryCursor<QueryV
 
   @inlinable
   public override func _element(sqliteStatement _: SQLiteStatement) throws -> Element {
-    let element = try QueryValue(decoder: &decoder).queryOutput
-    decoder.next()
-    return element
+    do {
+      let element = try QueryValue(decoder: &decoder).queryOutput
+      decoder.next()
+      return element
+    } catch QueryDecodingError.missingRequiredColumn {
+      let columnIndex = Int(decoder.currentIndex) - 1
+      throw DecodingError(
+        columnIndex: columnIndex,
+        columnName: _statement.columnNames[columnIndex],
+        sql: _statement.sql
+      )
+    }
   }
 }
 
@@ -62,9 +94,18 @@ final class QueryPackCursor<
 
   @inlinable
   public override func _element(sqliteStatement _: SQLiteStatement) throws -> Element {
-    let element = try decoder.decodeColumns((repeat each QueryValue).self)
-    decoder.next()
-    return element
+    do {
+      let element = try decoder.decodeColumns((repeat each QueryValue).self)
+      decoder.next()
+      return element
+    } catch QueryDecodingError.missingRequiredColumn {
+      let columnIndex = Int(decoder.currentIndex) - 1
+      throw DecodingError(
+        columnIndex: columnIndex,
+        columnName: _statement.columnNames[columnIndex],
+        sql: _statement.sql
+      )
+    }
   }
 }
 

Sources/StructuredQueriesGRDBCore/QueryCursor.swift

@@ -21,7 +21,7 @@ struct FetchAllTests {
       for index in 1...count {
         group.addTask {
           try await database.write { db in
-            try Record.insert(Record(id: index)).execute(db)
+            try Record.insert { Record(id: index) }.execute(db)
           }
         }
       }
@@ -43,6 +43,23 @@ struct FetchAllTests {
     try await $records.load()
     #expect(records == (0...(count/2-1)).map { Record(id: $0 * 2 + 1) })
   }
+
+  @Test func fetchFailure() {
+    do {
+      try database.read { db in
+        _ = try Record
+          .select { ($0.id, $0.date, #sql("\($0.optionalDate)", as: Date.self)) }
+          .fetchAll(db)
+      }
+      Issue.record()
+    } catch {
+      #expect("\(error)".contains(
+        """
+        Expected column 2 ("optionalDate") to not be NULL
+        """
+      )
+    }
+  }
 }
 
 @Table
@@ -60,15 +77,15 @@ extension DatabaseWriter where Self == DatabaseQueue {
       try #sql(
         """
         CREATE TABLE "records" (
-          "id" INTEGER PRIMARY KEY AUTOINCREMENT
-          , "date" INTEGER NOT NULL DEFAULT 42
-          , "optionalDate" INTEGER
+          "id" INTEGER PRIMARY KEY AUTOINCREMENT,
+          "date" INTEGER NOT NULL DEFAULT 42,
+          "optionalDate" INTEGER
         )
         """
       )
       .execute(db)
       for _ in 1...3 {
-        _ = try Record.insert(Record.Draft()).execute(db)
+        _ = try Record.insert { Record.Draft() }.execute(db)
       }
     }
     return database

Tests/SharingGRDBTests/FetchAllTests.swift

@@ -50,7 +50,7 @@ extension DatabaseWriter where Self == DatabaseQueue {
       )
       .execute(db)
       for _ in 1...3 {
-        _ = try Record.insert(Record.Draft()).execute(db)
+        _ = try Record.insert { Record.Draft() }.execute(db)
       }
     }
     try migrator.migrate(database)

Tests/SharingGRDBTests/FetchTests.swift

@@ -15,19 +15,19 @@ struct IntegrationTests {
     #expect(syncUps == [])
 
     try await database.write { db in
-      _ = try SyncUp.insert(SyncUp.Draft(isActive: true, title: "Engineering"))
+      _ = try SyncUp.insert { SyncUp.Draft(isActive: true, title: "Engineering") }
         .execute(db)
     }
     try await $syncUps.load()
     #expect(syncUps == [SyncUp(id: 1, isActive: true, title: "Engineering")])
     try await database.write { db in
-      _ = try SyncUp.upsert(SyncUp.Draft(id: 1, isActive: false, title: "Engineering"))
+      _ = try SyncUp.upsert { SyncUp.Draft(id: 1, isActive: false, title: "Engineering") }
         .execute(db)
     }
     try await $syncUps.load()
     #expect(syncUps == [])
     try await database.write { db in
-      _ = try SyncUp.upsert(SyncUp.Draft(id: 1, isActive: true, title: "Engineering"))
+      _ = try SyncUp.upsert { SyncUp.Draft(id: 1, isActive: true, title: "Engineering") }
         .execute(db)
     }
     try await $syncUps.load()
@@ -40,19 +40,19 @@ struct IntegrationTests {
     #expect(syncUps == [])
 
     try await database.write { db in
-      _ = try SyncUp.insert(SyncUp.Draft(isActive: true, title: "Engineering"))
+      _ = try SyncUp.insert { SyncUp.Draft(isActive: true, title: "Engineering") }
         .execute(db)
     }
     try await $syncUps.load()
     #expect(syncUps == [SyncUp(id: 1, isActive: true, title: "Engineering")])
     try await database.write { db in
-      _ = try SyncUp.upsert(SyncUp.Draft(id: 1, isActive: false, title: "Engineering"))
+      _ = try SyncUp.upsert { SyncUp.Draft(id: 1, isActive: false, title: "Engineering") }
         .execute(db)
     }
     try await $syncUps.load()
     #expect(syncUps == [])
     try await database.write { db in
-      _ = try SyncUp.upsert(SyncUp.Draft(id: 1, isActive: true, title: "Engineering"))
+      _ = try SyncUp.upsert { SyncUp.Draft(id: 1, isActive: true, title: "Engineering") }
         .execute(db)
     }
     try await $syncUps.load()