Finish custom constraints implementation

moor/lib/src/runtime/structure/columns.dart

@@ -27,8 +27,15 @@ abstract class GeneratedColumn<T, S extends SqlType<T>> extends Column<T, S> {
   /// [here](https://www.sqlite.org/syntax/column-def.html), into the given
   /// buffer.
   void writeColumnDefinition(StringBuffer into) {
-    into..write('${$name} $typeName ')..write($nullable ? 'NULL' : 'NOT NULL');
-    writeCustomConstraints(into);
+    into.write('${$name} $typeName ');
+
+    if ($customConstraints == null) {
+      into.write($nullable ? 'NULL' : 'NOT NULL');
+      // these custom constraints refer to builtin constraints from moor
+      writeCustomConstraints(into);
+    } else {
+      into.write($customConstraints);
+    }
   }
 
   @visibleForOverriding

moor/test/data/tables/todos.dart

@@ -26,7 +26,8 @@ class Users extends Table {
 @DataClassName('Category')
 class Categories extends Table {
   IntColumn get id => integer().autoIncrement()();
-  TextColumn get description => text().named('desc')();
+  TextColumn get description =>
+      text().named('desc').customConstraint('NOT NULL UNIQUE')();
 }
 
 class SharedTodos extends Table {

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

@@ -217,10 +217,9 @@ class $CategoriesTable extends Categories
       _id ??= GeneratedIntColumn('id', false, hasAutoIncrement: true);
   GeneratedTextColumn _description;
   @override
-  GeneratedTextColumn get description => _description ??= GeneratedTextColumn(
-        '`desc`',
-        false,
-      );
+  GeneratedTextColumn get description =>
+      _description ??= GeneratedTextColumn('`desc`', false,
+          $customConstraints: 'NOT NULL UNIQUE');
   @override
   List<GeneratedColumn> get $columns => [id, description];
   @override

moor/test/schema_test.dart

@@ -24,7 +24,7 @@ void main() {
           'category INTEGER NULL);'));
 
       verify(mockQueryExecutor.call('CREATE TABLE IF NOT EXISTS categories '
-          '(id INTEGER PRIMARY KEY AUTOINCREMENT, `desc` VARCHAR NOT NULL);'));
+          '(id INTEGER PRIMARY KEY AUTOINCREMENT, `desc` VARCHAR NOT NULL UNIQUE);'));
 
       verify(mockQueryExecutor.call('CREATE TABLE IF NOT EXISTS users '
           '(id INTEGER PRIMARY KEY AUTOINCREMENT, name VARCHAR NOT NULL, '

moor_generator/lib/src/writer/table_writer.dart

@@ -98,6 +98,11 @@ class TableWriter {
       }
     }
 
+    if (column.customConstraints != null) {
+      // todo should we worry about escaping this string?
+      additionalParams['\$customConstraints'] = "'${column.customConstraints}'";
+    }
+
     // GeneratedIntColumn('sql_name', isNullable, additionalField: true)
     final expressionBuffer = StringBuffer()
       ..write(column.implColumnTypeName)