sequelize.define('referral', {
        status: {
            type: DataTypes.ENUM('OPENED', 'INTERESTED'),
            allowNull: false,
            defaultValue: null
        },
        maxReferrals: {
            field: 'max_referrals',
            type: DataTypes.INTEGER,
            allowNull: false,
            defaultValue: null
        },
        properties: {
            type: DataTypes.JSON,
            allowNull: true,
            defaultValue: null
        },
        reward: {
            type: DataTypes.NUMERIC,
            allowNull: true,
            defaultValue: null
        },
        broken: {
            type: DataTypes.BOOLEAN,
            allowNull: false,
            defaultValue: false
        }
    }, {tableName: 'referral', underscored: true, underscoredAll: true});

Later on:
Referral.belongsTo(Referral, {foreignKey: 'referral_id', as: 'referral', constraints: false});