Custom Fields for Discount Codes

We extended the DiscountCode model to support custom fields. This allows users to categorize discount codes any way they want and fetch them using the predicates on the discount codes endpoint.

• [API] Added a custom field to the DiscountCode and DiscountCodeDraft.
• [API] Added Set Custom Type and Set CustomField update actions to manage the custom fields on a discount code.
• [API] Support searching for DiscountCodes with predicates like custom(field(field_name="field_value"))