Setup

Adding a basemap

Follow the steps in outlined in Lesson 1 to add Open Street Maps as a basemap image. A reminder video for this process is below. Make sure that the CRS changes to EPSG: 3857 to enable quick rendering of the basemap. By default QGIS projects data on-the-fly so other layers will be projected to the project CRS.

Creating a GeoPackage dataset

In the directory qgis_field_training create a new GeoPackage dataset called complex_form.gpkg with polygon geometry type. You can use the instructions in Lesson 1 for the process to create a new GeoPackage dataset. As a reminder watch the video below.

Setup Attribute Table

Follow the steps for Field Creation in Lesson 1 to create the following fields:

  • a numeric field of type integer called crop_cover_pct
  • a numeric field of type real called field_area
  • a text field of type text with 60 characters called unique_id

Use the following screengrabs as guides.

Create integer field

Create real field

Create unique ID field

We will use the unique_id field when we set up a one-to-many relation between two tables to allow us to collect multiple records per geometry feature.

Constraints

Now you have created your fields you need to create the attribute form. This process is outlined in the Simple Form Creation section of Lesson 1. Open the complex_form layer Properties and open up the Attribute Form widget.

Attributes Form widget

Next, set up the attribute form with a Drag and drop designer. The under Available Widgets select crop_cover_pct so it is highlighted grey. Give this field the alias % crop cover in field?. When you enter data into the field in the form you will be inputting the percentage of a field covered by crops. A field cannot have more than 100 % crop coverage; therefore, you need to set a constraint to prevent a data collector accidentally entering a value more than 100. To do this go to Constraints then enter the expression crop_cover_pct<=100 and give an Expression description: crop cover cannot be more than 100 % and check the checkbox Enforce expression constraint. By checking this checkbox you will not be able to enter data into this field unless the constraint expression is true. Here, you are preventing the form from being completed unless a number less than 100 is entered. If you try and enter a number greater than 100 the Expression description will appear.

Set expression constraint

There are a range of QGIS expressions that you can use to set up constraints in your form. You can find out about QGIS expressions here.

Area

You also want to be able to compute the area of the field. QGIS provides a range of functions that you can use to assist with tasks such as this; you can find a list of available functions here and you can also create your own functions. To compute the area of a field (i.e. a polygon feature that you will have digitized using QField) you can use the $area function which is part of the geometry functions.

To set up your form to compute the area select the field_area field in Available Widgets then give the field an alias area of field? and uncheck the Editable checkbox. Next, go to Defaults and enter $area. When you digitize a polygon in QField the value returned from executing the $area function will be assigned to field_area.

Set default to return result of area function

You will notice this process is similar to how you set up the survey_date field in simple_form. In that instance you used the $now function to return the date-time that the feature was created in QField.

One-to-many relationships

You can create a one-to-many relation between two tables when you need to collect multiple records (features) for a single geometry feature. This is a useful functionality for many field data collection activities; for example, you may need to fill out a form for each crop type grown in an intercropped field while a different number of crop types are grown in different fields. Using a one-to-many relation will allow you to fill out a form for each crop type in each field and each field can have a different number of crops cultivated in it.

This functionality is enabled in QGIS through establishing a one-to-many (1-N) relation between two tables. This is a database concept called Entity Relationship Diagramming. You have a table (table1) where each feature is identified by a primary key. Multiple features in another table (table2) can relate to single features in table1 through a foreign key (which takes on the relevant value of the primary key in table1). These concepts are explained further here and here.

To set up a one-to-many relation in QGIS you need to create a second table to relate to the attribute table of complex_form. To do this create a new GeoPackage layer; however, this time select the Database as complex_form.gpkg and choose the Geometry type as No geometry and the Table name as many_records. Select Add New Layer when the warning box appears.

Create table with no geometry in complex_form.gpkg

Having created your table many_records you need to add fields to it. To do this open the layer properties and then go to the Source Fields tab. Add the following fields to the table.

Add crop_type field

Add fertiliser field

Add foreign key field

Add Relation

Having created a foreign key and other fields in your many_records table you need to add the relation to the complex_form table. To do this go to the Project menu then Properties. In Project Properties select the Relations tab on the left-hand toolbar.

Project properties

Next, you need to add the relation between the complex_form table and the many_records table. Click on Add Relation and fill in the form as in the image below. You can find more instructions for filling out the Add Relation form here and defining one-to-many relationships.

Add relation

Now you have established a one-to-many relation between features in complex_form and features in many_records.

Finally, you need to set up the form to enter data into many_records. Go to the Properties then Attribute Form of many_records. First, setup the foreign key fk_field asa Relation Reference widget. Make sure you uncheck the Editable checkbox.

Foreign key as Relation Reference

Next, setup the crop_type field as a Value Map widget where you can select a crop from a drop-down list and give it the alias crop type>.

Crop type value map

Finally, setup the fertiliser field with a Checkbox widget.

Checkbox widget setup

The final step in setting up a one-to-many data collection form is to add the relation to the attribute form for complex_form. Go to the layer properties and Attribute Form tab for complex_form. First, you need to uncheck the Editable checkbox for the unique_id field and in Default enter uuid(). Then drag the relation named one_to_many from Relations under Available Widgets to the Form Layout as in the below image.

Add relation to attribute form

Package for QField

You have now set up a form with constraints and a one-to-many relation. Save the project as a .qgs file called complex_form.qgs then configure the project using QField Sync. Then package the project into the directory export_complex_form. Make sure you set your tables to offline editing during project configuration. Copy the export_complex_form folder to your mobile device.

One-to-many in action

Open up complex_form.qgs in QField. Check out the constraint set on the crop_cover_pct field and entering data into many_records using the value relation widget. If you have set up your constraint and the one-to-many relation successfully your form should function similar to the example in the video below.