python - QGIS 2.2 - possible to do a JOINS from the Action list?

I have a set of layers that need to be joined (Layer Properties > Joins) to one specific layer. The reason for this is that I will be updating and adding new layers into my project and would like a single Action command for the specific layer to automatically have the selected layers join with it. I want to avoid having to run joining/merging algorithms altogether.

Is this easily achievable?

After spending time searching on how to create Actions, most of what I found describe the same command which is how to open up a web browser to search for whatever is in the specified attribute. Unfortunately, the default actions don't seem to contain anything similar to what I am looking for.

Answer

Combining the answer of this previous question with the QGIS API results in python code that should do what you ask for.

#attach a QgsMapLayerAction to all layers:
joinLayerAction = qgis.gui.QgsMapLayerAction( "Join to layer", iface );
#add the action to the QGIS gui, so that it appears as an action for the layer
qgis.gui.QgsMapLayerActionRegistry.instance().addMapLayerAction(joinLayerAction)

def do_join(layer):
    #layer is a reference to the layer the actions was triggered on

    joinInfo = QgsVectorJoinInfo()

    joinInfo.joinLayerId = 'layer_id'        # TODO: insert correct id here
    joinInfo.joinFieldName = 'primary_key'   # TODO: insert correct key here
    joinInfo.targetFieldName = 'foreign_key' # TODO: insert correct key here
    joinInfo.memoryCache = True              # Tune to your needs

    layer.addJoin(joinInfo)

#connect to action trigger
joinLayerAction.triggeredForLayer.connect(do_join)

Just execute this code in the python console for testing. After that you can implement it as a python plugin or in a project python macro.