How to transform a multipart into a singlepart object in QGIS

Let's assume that we want to check if there are any multipart objects in our QGIS project. Let's start with the sample project:

We configure the Topology Checker:

In this example, we have two multipart objects.

To fix it:

Vectorial > Geoprocessing tools > Geometry tools > Multipart to singleparts

When we open the attribute table, we see the repeated field (there are multiple segments in an object). The goal is to transform the repeated field into a single element.

This segment is the one that is causing the problem (it has the first vertex of the geometry).

This segment was created first, but that means it doesn't have the first/last vertex at the beginning/end. We will separate the vertex lightly to create continuity:

At this point, we can dissolve the line, so that the attribute table has a single field:

Vectorial > Geoprocessing tools > Dissolve

Now, the line has the first vertex at the beginning:

And the last vertex at the end:

Finally, we need to edit the segment and leave it as it was originally:

We will use the magnet tool for that:

Here is the final result: