Avoid writing duplicate mapping methods

moor/test/data/tables/todos.g.dart

@@ -1270,16 +1270,6 @@ abstract class _$TodoDb extends GeneratedDatabase {
         }).map((rows) => rows.map(_rowToTodoEntry).toList());
   }
 
-  TodoEntry _rowToTodoEntry(QueryRow row) {
-    return TodoEntry(
-      id: row.readInt('id'),
-      title: row.readString('title'),
-      content: row.readString('content'),
-      targetDate: row.readDateTime('target_date'),
-      category: row.readInt('category'),
-    );
-  }
-
   Future<List<TodoEntry>> search(
       int id,
       {@Deprecated('No longer needed with Moor 1.6 - see the changelog for details')

moor_generator/lib/src/dao_generator.dart

@@ -68,8 +68,9 @@ class DaoGenerator extends GeneratorForAnnotation<UseDao> {
       buffer.write('$infoType get $getterName => db.$getterName;\n');
     }
 
+    final writtenMappingMethods = <String>{};
     for (var query in resolvedQueries) {
-      QueryWriter(query).writeInto(buffer);
+      QueryWriter(query, writtenMappingMethods).writeInto(buffer);
     }
 
     buffer.write('}');

moor_generator/lib/src/writer/database_writer.dart

@@ -60,8 +60,9 @@ class DatabaseWriter {
     }
 
     // Write implementation for query methods
+    final writtenMappingMethods = <String>{};
     for (var query in db.queries) {
-      QueryWriter(query).writeInto(buffer);
+      QueryWriter(query, writtenMappingMethods).writeInto(buffer);
     }
 
     // Write List of tables, close bracket for class

moor_generator/lib/src/writer/query_writer.dart

@@ -14,7 +14,9 @@ class QueryWriter {
   SqlSelectQuery get _select => query as SqlSelectQuery;
   UpdatingQuery get _update => query as UpdatingQuery;
 
-  QueryWriter(this.query);
+  final Set<String> _writtenMappingMethods;
+
+  QueryWriter(this.query, this._writtenMappingMethods);
 
   /// The expanded sql that we insert into queries whenever an array variable
   /// appears. For the query "SELECT * FROM t WHERE x IN ?", we generate
@@ -49,18 +51,23 @@ class QueryWriter {
   /// Writes a mapping method that turns a "QueryRow" into the desired custom
   /// return type.
   void _writeMapping(StringBuffer buffer) {
-    buffer
-      ..write('${_select.resultClassName} ${_nameOfMappingMethod()}')
-      ..write('(QueryRow row) {\n')
-      ..write('return ${_select.resultClassName}(');
+    // avoid writing mapping methods twice if the same result class is written
+    // more than once.
+    if (!_writtenMappingMethods.contains(_nameOfMappingMethod())) {
+      buffer
+        ..write('${_select.resultClassName} ${_nameOfMappingMethod()}')
+        ..write('(QueryRow row) {\n')
+        ..write('return ${_select.resultClassName}(');
 
-    for (var column in _select.resultSet.columns) {
-      final fieldName = _select.resultSet.dartNameFor(column);
-      final readMethod = readFromMethods[column.type];
-      buffer.write("$fieldName: row.$readMethod('${column.name}'),");
+      for (var column in _select.resultSet.columns) {
+        final fieldName = _select.resultSet.dartNameFor(column);
+        final readMethod = readFromMethods[column.type];
+        buffer.write("$fieldName: row.$readMethod('${column.name}'),");
+      }
+
+      buffer.write(');\n}\n');
+      _writtenMappingMethods.add(_nameOfMappingMethod());
     }
-
-    buffer.write(');\n}\n');
   }
 
   void _writeOneTimeReader(StringBuffer buffer) {