HStoreField lookups

# ======================================================================
# `HStoreField` is a flat string→string map. Query a key directly
# (`labels__env`), check a key exists (`__has_key`), or test containment
# (`__contains`). Requires the `hstore` extension (enabled in migration 0001).
# ======================================================================

# ----------------------------------------------------------------------
# labels.env = 'prod'
# ----------------------------------------------------------------------

# Django:
Server.objects.filter(labels__env="prod")

# SQL:
#   SELECT "examples_server"."id", "examples_server"."hostname", "examples_server"."labels"
#   FROM "examples_server"
#   WHERE ("examples_server"."labels" -> 'env') = 'prod'

# Result:
#   web-1

# ----------------------------------------------------------------------
# labels has_key 'region'
# ----------------------------------------------------------------------

# Django:
Server.objects.filter(labels__has_key="region")

# SQL:
#   SELECT "examples_server"."id", "examples_server"."hostname", "examples_server"."labels"
#   FROM "examples_server"
#   WHERE "examples_server"."labels" ? 'region'

# Result:
#   web-1
#   web-2

# ----------------------------------------------------------------------
# labels contains {'env': 'staging'}
# ----------------------------------------------------------------------

# Django:
Server.objects.filter(labels__contains={"env": "staging"})

# SQL:
#   SELECT "examples_server"."id", "examples_server"."hostname", "examples_server"."labels"
#   FROM "examples_server"
#   WHERE "examples_server"."labels" @> '"env"=>"staging"'::hstore

# Result:
#   web-2