diff --git a/angel_orm/CHANGELOG.md b/angel_orm/CHANGELOG.md
index f7f4c220..3a624ab8 100644
--- a/angel_orm/CHANGELOG.md
+++ b/angel_orm/CHANGELOG.md
@@ -1,3 +1,8 @@
+# 1.0.0-alpha+7
+* Added a `@belongsToMany` annotation class.
+* Resolved [#20](https://github.com/angel-dart/orm/issues/20). The
+`PostgreSQLConnectionPool` keeps track of which connections have been opened now.
+
 # 1.0.0-alpha+6
 * `DateTimeSqlExpressionBuilder` will no longer automatically
 insert quotation marks around names.
diff --git a/angel_orm/lib/src/pool.dart b/angel_orm/lib/src/pool.dart
index 9cdb0722..da77b854 100644
--- a/angel_orm/lib/src/pool.dart
+++ b/angel_orm/lib/src/pool.dart
@@ -38,7 +38,10 @@ class PostgreSQLConnectionPool {
     var connection = _connections[_index++];
     if (_index >= _connections.length) _index = 0;
 
-    if (!_opened.contains(connection.hashCode)) await connection.open();
+    if (!_opened.contains(connection.hashCode)) {
+      await connection.open();
+      _opened.add(connection.hashCode);
+    }
 
     return connection;
   }
diff --git a/angel_orm/lib/src/relations.dart b/angel_orm/lib/src/relations.dart
index 93677342..8eb57c26 100644
--- a/angel_orm/lib/src/relations.dart
+++ b/angel_orm/lib/src/relations.dart
@@ -2,6 +2,7 @@ abstract class RelationshipType {
   static const int HAS_MANY = 0;
   static const int HAS_ONE = 1;
   static const int BELONGS_TO = 2;
+  static const int BELONGS_TO_MANY = 3;
 }
 
 class Relationship {
@@ -24,7 +25,7 @@ class HasMany extends Relationship {
       String foreignKey,
       String foreignTable,
       bool cascadeOnDelete: false})
-      : super(0,
+      : super(RelationshipType.HAS_MANY,
             localKey: localKey,
             foreignKey: foreignKey,
             foreignTable: foreignTable,
@@ -39,7 +40,7 @@ class HasOne extends Relationship {
       String foreignKey,
       String foreignTable,
       bool cascadeOnDelete: false})
-      : super(1,
+      : super(RelationshipType.HAS_ONE,
             localKey: localKey,
             foreignKey: foreignKey,
             foreignTable: foreignTable,
@@ -51,10 +52,21 @@ const HasOne hasOne = const HasOne();
 class BelongsTo extends Relationship {
   const BelongsTo(
       {String localKey: 'id', String foreignKey, String foreignTable})
-      : super(2,
+      : super(RelationshipType.BELONGS_TO,
             localKey: localKey,
             foreignKey: foreignKey,
             foreignTable: foreignTable);
 }
 
 const BelongsTo belongsTo = const BelongsTo();
+
+class BelongsToMany extends Relationship {
+  const BelongsToMany(
+      {String localKey: 'id', String foreignKey, String foreignTable})
+      : super(RelationshipType.BELONGS_TO_MANY,
+            localKey: localKey,
+            foreignKey: foreignKey,
+            foreignTable: foreignTable);
+}
+
+const BelongsToMany belongsToMany = const BelongsToMany();
diff --git a/angel_orm/pubspec.yaml b/angel_orm/pubspec.yaml
index 9cbc0b23..2680fcce 100644
--- a/angel_orm/pubspec.yaml
+++ b/angel_orm/pubspec.yaml
@@ -1,5 +1,5 @@
 name: angel_orm
-version: 1.0.0-alpha+6
+version: 1.0.0-alpha+7
 description: Runtime support for Angel's ORM.
 author: Tobe O <thosakwe@gmail.com>
 homepage: https://github.com/angel-dart/orm