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Move canonical knowledge for Microsoft-owned review domains into the Microsoft layer and document the skill/knowledge co-location policy. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Co-authored-by: Jesper Schulz-Wedde <jesper.schulzwedde@microsoft.com> Copilot-Session: 2a6ea875-d38e-4f30-aadb-0d606f9be231
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xRec is a before-image in OnRename and OnDelete, but mirrors Rec in OnInsert and OnModify from code
Description
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.
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.
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.
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.
See also validate-table-relation-false-suppresses-rename-propagation.md, which describes when such a hand-written OnRename fix-up is required.
Best Practice
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.
See sample: xrec-is-a-before-image-only-in-some-triggers.good.al.
Anti Pattern
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.
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.
See sample: xrec-is-a-before-image-only-in-some-triggers.bad.al.