qgis - How to define map extents based on a feature's geometry?

There is a polygon geometry in my QGIS project which represents the extents of my map. Does anybody know how to use the data defined override to get the X/Y min and X/Y max coordinates of the polygon in the print composer?

Unfortunately this isn't working:

xmin(attribute(getFeature('layername', 'gid', 1), 'geom'))

Answer

The geometry can be accessed directly rather than through the attributes:

x_min(geometry(getFeature('airports', 'cat', 1)))