Finally I had to remove then recreate my column:
remove_column :messages, :area_shape
add_column :messages, :area_shape, :multi_polygon, srid: 3785
I think I can now understand why it's not possible. Indeed, it seems difficult to change a polygon
type to a multi_polygon
type without losing data logique.
If you really need to change the type, you can use what was said by @Mike (manually) and create a small method to convert polygon
to multi_polygon
but it's not really safe in my mind.
Tip: a multi_polygon
type is an Enumerable
that means multi_polygon
accepts Array
type.