--- title: "8: Computed and asserted fields | SurrealDB University" description: "COMPUTED duration, ASSERT on progress, and VALUE timestamps on the activity table." url: https://surrealdb.com/learn/schemas/page-08 --- ![Course content preview](https://surrealdb.com/assets/static/course-schemas.D4CFbBhP.avif) [Back to Courses](https://surrealdb.com/learn) Course chapters [Schema internals and migrations](https://surrealdb.com/learn/schemas) Internals [1: Schemaless vs. schemafull](https://surrealdb.com/learn/schemas/page-01) [2: Schema internals](https://surrealdb.com/learn/schemas/page-02) [3: Migrations](https://surrealdb.com/learn/schemas/page-03) [4: Data types](https://surrealdb.com/learn/schemas/page-04) [5: Automation](https://surrealdb.com/learn/schemas/page-05) Migrations [6: SurrealKit and the first table](https://surrealdb.com/learn/schemas/page-06) [7: Activities and seed data](https://surrealdb.com/learn/schemas/page-07) [8: Computed and asserted fields](https://surrealdb.com/learn/schemas/page-08) [9: Graph dependencies](https://surrealdb.com/learn/schemas/page-09) [10: Sync vs rollouts](https://surrealdb.com/learn/schemas/page-10) [11: Milestones](https://surrealdb.com/learn/schemas/page-11) [12: People and indexes](https://surrealdb.com/learn/schemas/page-12) [13: Events and CI](https://surrealdb.com/learn/schemas/page-13) [14: Capstone](https://surrealdb.com/learn/schemas/page-14) # 8: Computed and asserted fields Part [5](https://surrealdb.com/learn/schemas/page-05) introduced these field clauses in isolation. Here they land in committed schema files for the planning domain, matching the [project planning sample](https://surrealdb.com/docs/learn/schema-management/schema-design/sample-industry-schemas#project-planning) activity table. As you go through this chapter, add each `DEFINE` statement to `database/schema/activity.surql`. You'll apply all of them together with a single `surrealkit sync` at the end. ## COMPUTED duration Duration shouldn't be stored and maintained by application code when both endpoints already live on the record. ```surql DEFINE FIELD duration ON activity COMPUTED end - start; ``` `COMPUTED` fields are evaluated on read (and during projections that need them), not stored at write time. That avoids stale durations when someone updates `end` without touching a separate column. ```surql SELECT name, start, end, duration FROM activity; ``` ## ASSERT progress range Replace the bare `TYPE float` line for `progress` in `activity.surql` with a constrained range. Don't leave both definitions in the file: SurrealKit treats each `DEFINE FIELD` as one catalog object keyed by table and field name, so two `progress` lines produce duplicate metadata and can break a later `rollout`. Reach for a range `ASSERT` on numeric bounds, and a literal union when the set is categorical instead (see part [4](https://surrealdb.com/learn/schemas/page-04)). ```surql DEFINE FIELD progress ON activity TYPE float ASSERT $value IN 0.0..=1.0; ``` Invalid writes fail at the field layer: ```surql UPDATE activity:concrete SET progress = 1.5; -- Field assertion failure ``` For a custom message, use `ASSERT` with `THROW`, as in part [5](https://surrealdb.com/learn/schemas/page-05). ## VALUE for rolling timestamps (preview) The milestone table uses `VALUE` `time::now()` on `last_updated` in the docs sample. On `activity`, you might add: ```surql DEFINE FIELD updated_at ON activity VALUE time::now(); ``` `VALUE` runs on every create and update, which is useful for "last touched" columns. `DEFAULT` `time::now()` only fills in missing values on create, unless you add `ALWAYS`. Once the three edits above are in `activity.surql`, apply them with one sync: ```bash surrealkit sync --user root --pass secret --ns main --db main ``` Existing records pick up `COMPUTED` `duration` immediately on the next `SELECT`. `ASSERT` only applies to new writes; if legacy data already violates the range, backfill it before you tighten the field (part [3](https://surrealdb.com/learn/schemas/page-03)). ## Alter vs edit the file On a shared database, changing a field definition is a migration: - Local dev: edit `activity.surql` and `sync` - Staging/production: `rollout plan` → `start` → deploy → `complete` (parts [10](https://surrealdb.com/learn/schemas/page-10)–[11](https://surrealdb.com/learn/schemas/page-11)) `ALTER FIELD` in the shell and editing the `.surql` file should describe the same end state. ## Checkpoint | Clause | Role on `activity` | | --- | --- | | `COMPUTED` `duration` | Derived from `start` / `end` | | `ASSERT` on `progress` | Enforces 0.0..=1.0 | | `VALUE` `updated_at` | Optional audit column | Next: model the dependency graph between activities and projects with `RELATION` tables. Previous 7: Activities and seed data [Previous](https://surrealdb.com/learn/schemas/page-07) Next lesson 9: Graph dependencies [Next lesson](https://surrealdb.com/learn/schemas/page-09) ```json {"@context":"https://schema.org","@type":"Course","name":"Schema internals and migrations","description":"Learn how SurrealDB stores schema metadata, how DEFINE statements shape your database, and how to migrate production data safely.","url":"https://surrealdb.com/learn/schemas","inLanguage":"en","isAccessibleForFree":true,"provider":{"@type":"Organization","name":"SurrealDB","url":"https://surrealdb.com"},"hasPart":[{"@type":"LearningResource","name":"Schema internals and migrations","url":"https://surrealdb.com/learn/schemas"},{"@type":"LearningResource","name":"1: Schemaless vs. schemafull","url":"https://surrealdb.com/learn/schemas/page-01"},{"@type":"LearningResource","name":"2: Schema internals","url":"https://surrealdb.com/learn/schemas/page-02"},{"@type":"LearningResource","name":"3: Migrations","url":"https://surrealdb.com/learn/schemas/page-03"},{"@type":"LearningResource","name":"4: Data types","url":"https://surrealdb.com/learn/schemas/page-04"},{"@type":"LearningResource","name":"5: Automation","url":"https://surrealdb.com/learn/schemas/page-05"},{"@type":"LearningResource","name":"6: SurrealKit and the first table","url":"https://surrealdb.com/learn/schemas/page-06"},{"@type":"LearningResource","name":"7: Activities and seed data","url":"https://surrealdb.com/learn/schemas/page-07"},{"@type":"LearningResource","name":"8: Computed and asserted fields","url":"https://surrealdb.com/learn/schemas/page-08"},{"@type":"LearningResource","name":"9: Graph dependencies","url":"https://surrealdb.com/learn/schemas/page-09"},{"@type":"LearningResource","name":"10: Sync vs rollouts","url":"https://surrealdb.com/learn/schemas/page-10"},{"@type":"LearningResource","name":"11: Milestones","url":"https://surrealdb.com/learn/schemas/page-11"},{"@type":"LearningResource","name":"12: People and indexes","url":"https://surrealdb.com/learn/schemas/page-12"},{"@type":"LearningResource","name":"13: Events and CI","url":"https://surrealdb.com/learn/schemas/page-13"},{"@type":"LearningResource","name":"14: Capstone","url":"https://surrealdb.com/learn/schemas/page-14"}]} ``` ```json {"@context":"https://schema.org","@type":"LearningResource","name":"8: Computed and asserted fields","description":"COMPUTED duration, ASSERT on progress, and VALUE timestamps on the activity table.","url":"https://surrealdb.com/learn/schemas/page-08","learningResourceType":"lesson","isPartOf":{"@type":"Course","name":"Schema internals and migrations","url":"https://surrealdb.com/learn/schemas"},"position":11} ``` ```json {"@context":"https://schema.org","@type":"Organization","name":"SurrealDB","url":"https://surrealdb.com","logo":"https://surrealdb.com/assets/static/logo.BG7_TG2b.svg","description":"SurrealDB is the unified data layer for AI. 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