async getCouponForOffer(offerId) {
        const row = await this.OfferModel.where({id: offerId}).query().select('stripe_coupon_id', 'discount_type').first();
        if (!row || row.discount_type === 'trial') {
            return null;
        }
        if (!row.stripe_coupon_id) {
            const offer = await this.offersAPI.getOffer({id: offerId});
            await this.createCouponForOffer(offer);
            return this.getCouponForOffer(offerId);
        }
        return {
            id: row.stripe_coupon_id
        };
    }
@param {string} offerId

@returns {Promise<{id: string}>}