How-to

Add a validation rule or computed field

A validation rule is a condition every record of an entity must meet, checked by every record write; a computed field is a field whose value is calculated. Both are designed on the entity's page in the Metadata Designer and belong to whoever owns the entity.

For: Builder, Platform administrator, Tenant administrator

Prerequisite

metadata.write at the scope you design in — the platform for a platform entity, the tenant for its own.
The entity and its fields; for a computed field, the field that will hold the value (add it under Fields first).

Steps

Open Metadata Designer and select the entity.
Under Rules, give a key and a name, choose the field, the test (is, is not, is one of) and the value a record must meet, what happens when it does not (error, warning or info) and the message the person sees, and press Add rule.
Under Computed fields, choose the field that holds the value, the calculation (join text, add, subtract, multiply, divide, or count or sum over a relation) and its parts, and the mode (virtual or materialized), and press Save computed field.

Result

Every record write — creating or editing in Explorer, a form, an action that sets fields, a workflow's validation step — checks the rule before anything is written, and a refusal names the rule. A platform entity's rules apply in every tenant that reads it. A package that carries the entity carries its rules and computed fields.

Keep in mind One condition per rule for now; add another rule for another condition. Explorer's record view does not show computed values yet, and a materialized computed field is not yet recomputed automatically when the fields it reads change.

Tip A computed field cannot read its own value, and a chain of computed fields that loops or runs too deep is refused with the reason.