A.associate = function(models) {
    A.belongsToMany(models.B, {
        through: 'A_B',
        as: 'Bs',
        foreignKey: 'a_id'
    });
};

C.associate = function(models) {
    C.hasMany(models.A_B, {
        as: 'ABs',
        foreignKey: 'c_id'
    });
};

Error: C.hasMany called with something that's not a subclass of Sequelize.Model