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The same plain-backtick "See sample: \`x.good.al\`." form fixed on al-methods-limited-during-write-transactions (PR #161) turned up repo-wide on 15 more of this PR's articles - Knowledge-Retrieval.ps1 requires the markdown-link form to associate a sample with its article. All 16 fixed; the four local validators (frontmatter, knowledge-index, knowledge-retrieval, review-fixtures, skill-index) pass.
28 lines
1.7 KiB
Markdown
28 lines
1.7 KiB
Markdown
---
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bc-version: [all]
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domain: web-services
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keywords: [api-page, derived-fields, exposure, odata]
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technologies: [al]
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countries: [w1]
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application-area: [all]
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---
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# Recalculate Stored Derived Fields Before Exposing Them on API Pages
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> Contributions welcome — open a PR to refine or extend this article.
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## Description
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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 Exposing such a field directly on an API page hands external consumers a snapshot that may be significantly out of date.
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## Best Practice
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Recalculate the derived value in `OnAfterGetRecord` from its authoritative source — typically a FlowField — using a page-level variable, and expose that recalculated value instead of the stale stored field. Whether to also expose the source fields is a separate design decision, not a requirement of this pattern; keep the API contract scoped to what consumers actually need. If letting the consumer verify the recalculation is itself a requirement, expose every field the calculation reads, not just one of them — a derived value with two inputs needs both exposed, or the "verification" is incomplete.
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See sample: [`stored-derived-fields-must-not-be-exposed-directly.good.al`](stored-derived-fields-must-not-be-exposed-directly.good.al).
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## Anti Pattern
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Exposing the stored field directly via `Rec`, trusting that it was kept in sync by whichever trigger last touched it.
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See sample: [`stored-derived-fields-must-not-be-exposed-directly.bad.al`](stored-derived-fields-must-not-be-exposed-directly.bad.al).
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