Advanced Form Configuration

• Form layout
  • Tabs and groups
  • Show and hide fields depending on a field value
• Default values
• Constraints
• Drill-down forms

In addition to the edit widgets, extra configuration can be done to the fields and form layout to make the data collection easier and more consistent.

Form layout

If your form contains more than a handful of fields, it becomes difficult to navigate and enter or view the data. Therefore it can be helpful to arrange fields in tabs and groups in the form.

TIP

Clone documentation/form_setup (opens new window) to follow this example!

Tabs and groups

To set the form layout, we are going to use the Drag-n-Drop designer.

1. Right-click on survey layer and select Properties
2. In the Attributes form tab, select Drag and Drop Designer
3. In Form Layout, we can create tabs and groups to arrange fields in the form. Fields can be moved from the left column to form layout. Only fields that are in the Form layout will appear in the form.

Here, we created two tabs, Information and Data, and a group Location inside the Information tab.

The form with tabs and groups and conditional visibility will appear in QGIS like this:

And this is how the same form looks like in Mergin Maps mobile app:

Show and hide fields depending on a field value

We can set the visibility of Location tab depending on the value of the survey field:

• If survey is set to False, the Location tab will not appear in the form,
• If survey is set to True, the Location tab will show up in the form.

1. Select the Location tab in the Form Layout column
2. Check the Control Visibility by Expression option
3. Type the following expression in the box below: "survey" IS TRUE
4. Background colour can be set in the Style tab.
   

Default values

Default values can be used to automatically record, e.g, the name of the surveyor, date and time of the survey, latitude and longitude of the feature, or to have frequently used values filled in advance.

TIP

Clone documentation/form_setup (opens new window) to follow this example!

The default value can be a text, number or a QGIS expression. Here we are going to set the default values for x and y field to be longitude and latitude.

1. Right-click on survey layer and select Properties
2. In the Attributes form tab, select the x field in the Available Widgets column on the left
3. In the Widget Display, set the Alias to Longitude
4. In Defaults tab, define the Default value as:
   x( transform( $geometry, 'EPSG:3857', 'EPSG:4326'))
   Note that the expression transforms the point from EPSG:3857 (map and layer coordinate reference system) to EPSG:4326.
5. Check the Apply default value on update

Repeat the same steps for y field and setting the Alias to Latitude and the default value to
y( transform( $geometry, 'EPSG:3857', 'EPSG:4326'))

Don't forget to save and synchronise your project!

When you survey a new point in Mergin Maps mobile app, you will see the values are automatically filled in:

Constraints

When collecting data, you may want to apply constraints to certain field(s) to avoid mistakes when the values are filled in in the field.

TIP

Clone documentation/form_setup (opens new window) to follow this example!

Here, we will set up a constraint to the number field that represents the number of plants.

1. Right-click on survey layer and select Properties
2. In the Attributes form tab, select the number field in the Available Widgets column on the left
3. In the Widget Display, define the Constraints:
   • check the Not null and Enforce not null constraints
   • Expression type "number" >= 1 and check the Enforce expression constraint option

When surveying new features in Mergin Maps mobile app, this field will have to be filled in using a value higher than or equal to 1. Otherwise, you will get a warning.

Drill-down forms

Cascade or drill-down forms enable to list values in a field depending on a value selected in another field.

TIP

Clone documentation/form_cascade (opens new window) to follow this example!

In this project, we will work with the point layer named car. The survey tab in the form contains information about cars, such as Car-Type, Car-Manufacturer, or Car-Model. These attributes refer to non-spatial tables using value relations.

To make the survey easier, we will set the drill-down form so that the values in the drop-down menus are filtered based on the previously selected values. For example, if Car type (truck/car): Trucks is selected, the Car-Manufacturer field will offer only truck manufacturers DAF Trucks, Scania, MAN. Likewise, the next drop-down menu for Car-Model will change accordingly based on the Car-Manufacturer selection, showing only relevant car models.

To set the drill-down form:

1. Right-click on the car layer and select Properties.

2. Navigate to the Attributes form tab. We will set the value relations for the Car-Type, Car-manufacturer, Car-model fields, which are located in the Survey tab of the form.

3. The Car-Type field refers to the car-type table. The key column is the fid and the value column is the name field:
   

   The car-type table is structured as follows:
   

   When entering data into this field, the user can choose from these four name values.
   

4. The Car-manufacturer field refers to the car-manufacturer table. This table contains the fields fid, name, and type.
   

   • Set the fid field as the key column and name as the value column.
   • Use the type field to define which values will be shown in the form based on the selected Car-type by entering this Filter expression:
     "type" = current_value('Car-Type')

Now, the form will offer only options, where the Type field of the car-manufacturer table matches the current value of the Car-type. When entering data, we will get a drill-down form: since Truck is selected as the Car type, only three manufacturers are available in the Car-Manufacturer field.

5. Car-model works similarly. It refers to the car-model table, limiting options based on the Car-manufacturer field.
   • Set the fid field as the key column and name as the value column.
   • Use the manufacturer field to filter the values using this Filter expression:
     "manufacturer" = current_value('Car-Manufacturer')

When doing the survey Mergin Maps mobile app, you will see that after selecting Car-type: Car, the Manufacturer field will only offer Mercedes-Benz, Skoda and Audi. After selecting Audi, the Car-Model field will only offer the appropriate car models.