Solved with the amazing scuttle.io:
Drawing.create(
:the_geom => Arel::Nodes::NamedFunction.new(
'ST_Transform', [
Arel::Nodes::NamedFunction.new(
'ST_SetSRID', [
Arel::Nodes::NamedFunction.new('ST_GeomFromText', ['foobar']), 4326
]
), 900913
]
)
)