module.exports = (sequelize) => {
    const { User, Group, FriendRequest, Friendship } = sequelize.models;

    // User <-> Group: Many-to-Many
    // A User can be in many Groups, and a Group can have many Users (members).
    const UserGroup = sequelize.define('UserGroup', {}, { timestamps: false });
    User.belongsToMany(Group, { through: UserGroup, foreignKey: 'userId' });
    Group.belongsToMany(User, { through: UserGroup, as: 'members', foreignKey: 'groupId' });

    // Friendship is a direct link between two users.
    // user1Id will always be less than user2Id to ensure uniqueness.
    Friendship.belongsTo(User, { as: 'user1', foreignKey: 'user1Id' });
    Friendship.belongsTo(User, { as: 'user2', foreignKey: 'user2Id' });

    // User <-> FriendRequest: One-to-Many
    // A User can send many friend requests and receive many friend requests.
    User.hasMany(FriendRequest, { as: 'sentRequests', foreignKey: 'fromUserId', onDelete: 'CASCADE' });
    User.hasMany(FriendRequest, { as: 'receivedRequests', foreignKey: 'toUserId', onDelete: 'CASCADE' });
    FriendRequest.belongsTo(User, { as: 'fromUser', foreignKey: 'fromUserId' });
    FriendRequest.belongsTo(User, { as: 'toUser', foreignKey: 'toUserId' });
};