from django.contrib.gis.gdal import SpatialReference

from django.db import connections, DEFAULT_DB_ALIAS

def add_srs_entry(srs, auth_name='EPSG', auth_srid=None, ref_sys_name=None,

                  database=DEFAULT_DB_ALIAS):

    """

    This function takes a GDAL SpatialReference system and adds its information

    to the `spatial_ref_sys` table of the spatial backend.  Doing this enables

    database-level spatial transformations for the backend.  Thus, this utility

    is useful for adding spatial reference systems not included by default with

    the backend -- for example, the so-called "Google Maps Mercator Projection"

    is excluded in PostGIS 1.3 and below, and the following adds it to the

    `spatial_ref_sys` table:

    >>> from django.contrib.gis.utils import add_srs_entry

    >>> add_srs_entry(900913)

    Keyword Arguments:

     auth_name:

       This keyword may be customized with the value of the `auth_name` field.

       Defaults to 'EPSG'.

     auth_srid:

       This keyword may be customized with the value of the `auth_srid` field.

       Defaults to the SRID determined by GDAL.

     ref_sys_name:

       For SpatiaLite users only, sets the value of the the `ref_sys_name` field.

       Defaults to the name determined by GDAL.

     database:

      The name of the database connection to use; the default is the value

      of `django.db.DEFAULT_DB_ALIAS` (at the time of this writing, it's value

      is 'default').

    """

    connection = connections[database]

    if not hasattr(connection.ops, 'spatial_version'):

        raise Exception('The `add_srs_entry` utility only works '

                        'with spatial backends.')

    if connection.ops.oracle or connection.ops.mysql:

        raise Exception('This utility does not support the '

                        'Oracle or MySQL spatial backends.')

    SpatialRefSys = connection.ops.spatial_ref_sys()

    # If argument is not a `SpatialReference` instance, use it as parameter

    # to construct a `SpatialReference` instance.

    if not isinstance(srs, SpatialReference):

        srs = SpatialReference(srs)

    if srs.srid is None:

        raise Exception('Spatial reference requires an SRID to be '

                        'compatible with the spatial backend.')

    # Initializing the keyword arguments dictionary for both PostGIS

    # and SpatiaLite.

    kwargs = {'srid' : srs.srid,

              'auth_name' : auth_name,

              'auth_srid' : auth_srid or srs.srid,

              'proj4text' : srs.proj4,

              }

    # Backend-specific fields for the SpatialRefSys model.

    if connection.ops.postgis:

        kwargs['srtext'] = srs.wkt

    if connection.ops.spatialite:

        kwargs['ref_sys_name'] = ref_sys_name or srs.name

    # Creating the spatial_ref_sys model.

    try:

        # Try getting via SRID only, because using all kwargs may

        # differ from exact wkt/proj in database.

        sr = SpatialRefSys.objects.get(srid=srs.srid)

    except SpatialRefSys.DoesNotExist:

        sr = SpatialRefSys.objects.create(**kwargs)

# Alias is for backwards-compatibility purposes.

add_postgis_srs = add_srs_entry