How to Link Multiple Records to One Feature

It is often the case that you have a set of spatial features and you want to record their status every now and then.

For example, there is a GIS layer representing the manholes and the surveyors carry out regular inspections of the manholes using Mergin Maps. Instead of duplicating the manhole layer and recording each inspection as a new feature, inspections can be recorded in a non-spatial table that is linked to the spatial layer. This way, multiple records can be linked to one feature as a one to many relation.

The image below shows the manhole locations and a form with listed inspections in Mergin Maps mobile app.

Example project available

Clone our public project documentation/forms_one-to-many-relations to follow this setup. The project includes a data sample.

The manhole point layer has the following attribute table:

fid	Manhole	Manhole uuid (uuid)
1	1	{db908450-4265-4070-a3f7-1d73705df8bf}
2	2	{aeea4c7d-e7c5-4638-8207-53fa469ccddd}
3	3	{042a715a-73ac-4bff-9bb0-7f598d216ec9}

This layer contains only information about the manholes. Manhole uuid (uuid) values are unique values generated using the uuid() function when a feature is created. This field will be used to link inspections and manholes.

Using UUID

Why UUID? FID can be changed during synchronisation. As a result, records can end up being linked to wrong features.

On the other hand, UUID (Universally Unique Identifier) is generated to be unique and will not be changed when synced. Therefore, we recommend always using UUID to link layers.

The non-spatial inspections table has the following attributes. Manhole uuid (parent-uuid) are references to manholes.

fid	Date	Blocked?	Flooded?	Inspector	Manhole uuid (parent-uuid)
8	2025-11-20	false	false	Joe Schmoe	{db908450-4265-4070-a3f7-1d73705df8bf}
9	2025-11-28	false	true	Fred Bloggs	{db908450-4265-4070-a3f7-1d73705df8bf}
10	2025-12-01	true	true	Fred Bloggs	{aeea4c7d-e7c5-4638-8207-53fa469ccddd}
11	2025-12-03	false	false	Joe Schmoe	{042a715a-73ac-4bff-9bb0-7f598d216ec9}

To configure 1-N relations in QGIS (detailed steps can be found in Relations):

1. From the main menu, select Projects > Properties ...

2. In the Relations tab, select Add Relation

3. Define the parent and child layers and the fields to link the two layers:

   • Name is the name of the relation, e.g. Inspection
   • Referenced (parent) is the spatial layer manhole_locations
   • Field 1 of the Referenced (parent) is the field Manhole UUID that contains the unique UUID
   • Referencing (child) is the non-spatial layer inspections
   • Field 1 of the Referencing (child) layer is the Manhole UUID, which acts as a foreign key to link inspections to spatial features

   

4. Navigate to the Attributes form tab in Layer Properties of the survey layer (manhole_locations)

5. Add the Inspections relation to the Form Layout

Now you can add multiple inspections for each manhole location. The inspections records will be stored in the inspections table.

When you open the form for an existing record in the manhole_locations point layer, it will display existing inspection records and you can also add, delete or edit the records:

In the mobile app, the form will display all linked inspection records. Tapping the + button opens the inspection form and a new inspection record can be added.