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Three related concerns, each proven by executable tests rather than recall. They were extracted from a real defect that shipped through a six-reviewer panel undetected, which is the admission test passing on behavior rather than on theory. owning-table-must-delete-dependents-in-ondelete AL has no cascading delete. What makes it missable is an asymmetry: the platform DOES keep references correct on rename via TableRelation, so a developer who learns that and generalizes it to delete ships orphans. Also notes the permission trap (delete rights needed on the dependent table, not just the parent). validate-table-relation-false-suppresses-rename-propagation The non-obvious half. The property name implies input validation only, but disabling it also switches off rename propagation. Verified against a parent renamed once while a child held three fields: a normal relation (follows), the same relation with validation disabled (does NOT follow), and a field with no relation at all (does not follow) — the third being the control that proves the test can detect a non-propagating field. xrec-is-a-before-image-only-in-some-triggers Corrects both the naive belief that xRec is always the previous record and the folk rule that it 'only works from a page'. The behavior is per-trigger: a genuine before-image in OnRename and OnDelete regardless of driver, a mirror of Rec in OnInsert/OnModify when driven from code, and a real before-image in those two only when a page drove the write. That last asymmetry is why an OnModify comparison against xRec passes manual page testing and silently no-ops in a job queue. Targets /community per CONTRIBUTING — general BC knowledge, not fork-specific. Frontmatter validator clean; Test-ReviewFixtures passes (32 cases, 16 leaves).
36 lines
2.4 KiB
Markdown
36 lines
2.4 KiB
Markdown
---
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bc-version: [all]
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domain: data-modeling
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keywords: [xrec, before-image, onmodify, onrename, oninsert, ondelete, table-trigger, page-driven]
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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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# `xRec` is a before-image in `OnRename` and `OnDelete`, but mirrors `Rec` in `OnInsert` and `OnModify` from code
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## Description
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`xRec` is widely believed to be "the previous record" inside every table trigger, and — in reaction to that — is often dismissed with the folk rule *"`xRec` only works from a page, never from code"*. Both are wrong, and the second is wrong in the place it matters most.
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The behaviour is per-trigger. In `OnRename` and `OnDelete`, `xRec` is a genuine before-image regardless of what drove the write. In `OnInsert` and `OnModify`, a **code-driven** write leaves `xRec` mirroring `Rec` — there is no before-image at all — while a **page-driven** write does supply one.
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That combination produces a defect that is unusually hard to catch. A comparison such as `if Rec.Status <> xRec.Status then` inside `OnModify` works when a tester clicks through a page, and silently never fires when the same code path runs from a job queue, a batch routine, or an API call. It fails as a no-op, not as an error.
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The `OnRename` case is the useful half: because `xRec` there holds the previous primary key even from code, it is the one place a hand-written key fix-up is reliable. Note that in `OnRename` only the key differs between `Rec` and `xRec` — non-key field values are identical on both sides.
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See also `validate-table-relation-false-suppresses-rename-propagation.md`, which describes when such a hand-written `OnRename` fix-up is required.
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## Best Practice
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Use `xRec` for the previous key in `OnRename`, and for the record being removed in `OnDelete`. In `OnModify`, obtain the before-image by re-reading the stored row rather than trusting `xRec`, so the logic behaves identically whether a page, a job queue or an API drove the write.
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See sample: `xrec-is-a-before-image-only-in-some-triggers.good.al`.
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## Anti Pattern
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Comparing `Rec` against `xRec` inside `OnModify` (or `OnInsert`) to detect a change. From code the two are equal, so the branch is dead and whatever it guards never happens.
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Detection signal: any read of `xRec` inside `OnModify` or `OnInsert`. Treat "but it works when I test it on the page" as confirmation of the defect rather than a refutation.
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See sample: `xrec-is-a-before-image-only-in-some-triggers.bad.al`.
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