Relation slowness and incorrect constraint state in Mergin Maps

[joonaris]

When adding data via a relation (parent–child) in Mergin Maps, the UI shows an incorrect validation error for a field with a unique constraint, even though the entered value is actually unique and the feature can be saved successfully.

The issue appears when creating a new related (child) feature from the parent feature using the Relation Editor. The constrained field remains marked as invalid (red / error state), causing confusion. Despite the error indication, Mergin Maps still allows the feature to be saved. Sometimes the validation error goes by

This results in a mismatch between the UI validation state and the actual database constraint enforcement.

Sometimes the constraint warning message disappears after waiting for a short while.
In the Mergin Maps Windows version and in QGIS, everything works smoothly without any noticeable lag.

Environment and Application (+ app version, build, operating system)
Mergin Maps v.2026.1.0
Mergin Maps QGIS plugin version 2025.4.0
QGIS 3.40.10.

Samsung Galaxy A52s
Android 14
On UI-versio 6.1.

Steps to reproduce

1. Open a Mergin Maps project with a parent–child relation defined.
2. Ensure the child table has a field (e.g. kohdeluokka --> class) with a unique constraint.
3. Select a parent feature.
4. Open the Relation Editor.
5. Create a new related (child) feature.
6. Enter a value into the constrained field that is unique.

Actual results

• The constrained field remains marked as invalid (red / constraint error).
• The validation error does not disappear, even though the value is unique.
• Mergin Maps allows saving the feature, despite the UI showing a validation error.
• UI validation state does not reflect the actual database state

Expected results

• When a unique value is entered:
  the validation error should disappear immediately
• UI validation should accurately reflect database constraints.
• Relation-based editing should not introduce noticeable performance issues.

Mergin_Maps_relation_slowness.mp4

[tomasMizera]

Hi @joonaris, thank you for reporting the issue and sorry to hear about the trouble! I forwarded the ticket to our support team, they will reply to get some more details.