Using QGIS Expression String Builder to display selected multiple layers in a map in print composer

Question

In QGIS, in a Map's item properties, inside print composer, there is an option to "lock layers" (display layers) based on an expression, by using QGIS "expression string builder".

I can use as expression the name of a layer in my QGIS project, and that layer will display in the map in the print composer. I'm sure I am missing some basic operator, but how can I display more than one layer? Which function or operator do I need to use?

Answer 1

The answer lies in the documentation.

The expression must be similar to this:

concat ('layer 1', '|', 'layer 2')

In QGIS 3.2, you will now see the expected format for any expression. Read more in the changelog.