Set default value based on multiple fields in QGIS

Question

I have a layer with a relate table (1-M) with three columns (fk_id, date, comment).

fid	fk_id	date	comment
1	01	2021年02月03日	Comment 1
2	02	2021年02月05日	Comment 2
3	02	2021年03月02日	Comment 3
4	02	2021年03月25日	Comment 4
5	03	2021年01月11日	Comment 5

I have also created a custom form for the layer and table.

Custom form

What I need is when I add a new record to the related table, the default comment must be the same as the last date for that fk_id.

So, in this example, if I create a new record in the related table using the form, the comment should be Comment 4.

I'm looking around the expression builder but I have no idea how to do this selection.

Anyone could give me a tip of how can I set this default?

• Erik: Just to check I got this right: Based on your ID, you want to grab the comment of the newest of the already existing features with the same ID?
• Raúl Casado: That's right (I guess I didn't explain myself well in the question).
• Erik: It's always tricky to explain familiar issue to others ;-) anyway, to my knowledge it is not possible to automatically base one attribute of a new feature on something you're entering while creating that feature.

Answer 1

To add a new record to the related table, with the default comment the same as the last date for that attribute, please set up some defaults through RMC > Properties > Attribute Form > Fields > date > Defaults

settings

I set map_get(attributes(get_feature_by_id(@layer, maximum("fid"))), 'date') as the 'Default value' as well as ticked the 'Apply default value on update' and clicked Apply.

So, now after adding a new feature it will possess the date of the last feature.

Mind that maximum() function also includes group_by argument.

For the example of the question, a working expression could be:

map_get(attributes(get_feature_by_id(@layer, maximum("fid", group_by:="fk_id"))), 'comment')

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References:

• How to set a default value for attributes in shapefiles in QGIS?
• QGIS Training Manual » 5.3.3. Follow Along: Setting Form Field Types

• Raúl Casado: Thanks Taras, I used your expression using group_by for maximum(): map_get(attributes(get_feature_by_id(@layer, maximum(fid, fk_id))), 'comment')