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Add 18 community AL/BC patterns across style, data-modeling, web-services, appsource, breaking-changes, performance, and testing

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Contributed by CURABIS ApS, generalized from patterns observed across real AppSource/PTE development. Each article follows the knowledge file format (frontmatter, Description/Best Practice/Anti Pattern, sibling .good.al/.bad.al samples).
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microsoft/knowledge/web-services/stored-derived-fields-must-not-be-exposed-directly.md Normal file
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---
bc-version: [all]
domain: web-services
keywords: [api-page, derived-fields, exposure, odata]
technologies: [al]
countries: [w1]
application-area: [all]
---
# Recalculate Stored Derived Fields Before Exposing Them on API Pages
> Contributions welcome — open a PR to refine or extend this article.
## Description
A stored field whose value is derived from other fields inside an `OnValidate` trigger only updates when that specific trigger fires. If the underlying source data changes through some other path, the stored value goes stale without raising any error. Exposing such a field directly on an API page hands external consumers a snapshot that may be significantly out of date.
## Best Practice
Recalculate the derived value in `OnAfterGetRecord` from its authoritative source — typically a FlowField — using a page-level variable, and expose both the recalculated value and the source field so the consumer can verify it.
See sample: `stored-derived-fields-must-not-be-exposed-directly.good.al`.
## Anti Pattern
Exposing the stored field directly via `Rec`, trusting that it was kept in sync by whichever trigger last touched it.
See sample: `stored-derived-fields-must-not-be-exposed-directly.bad.al`.