Comparing values between two fields in QGIS Graphical Modeler

Question

I'm building a graphical model in QGIS 3.4 to check if a point feature with a specific value in the A field is within a polygon with the same value in B field.

The image displays the draft of the model. I set two vector layers as input (point layer and polygon layer), then I added the algorithm "Join attributes by location". After I tried to use "Select by attribute" to compare the attribute values between two fields and I continued adding "Extract selected feature" algorithm.

My issue is in "Select by attribute" part, I don't know what I need to write in the "Value" field or if there are other solutions to compare the value of the two fields.

I found this post that uses actions but I prefer Graphical Modeler.

Answer 1

Since you need to use QGIS 3.4, I'll stay in the basic modeler. It can be as simple not using the selection algorithms, but rather the Extract by expression algorithm. Selecting features in a model doesn't always work perfectly and since creating new layers in memory doesn't leave a huge mess behind, extracting directly and using that extraction in subsequent algorithms usually works exactly the same way. Only what you actively save to a temporary layer or a file will remain. For your model, you can replicate this one with the following parameters for the Extract:

• Input layer: The result of Join attributes by location
• Expression: "FieldA" = "FieldB"
• Matching features: The layer you want to save

It shouldn't be more complicated than that.