from django.db.backends.postgresql_psycopg2.introspection import DatabaseIntrospection

from django.contrib.gis.gdal import OGRGeomType

class GeoIntrospectionError(Exception):

    pass

class PostGISIntrospection(DatabaseIntrospection):

    # Reverse dictionary for PostGIS geometry types not populated until

    # introspection is actually performed.

    postgis_types_reverse = {}

    def get_postgis_types(self):

        """

        Returns a dictionary with keys that are the PostgreSQL object

        identification integers for the PostGIS geometry and/or

        geography types (if supported).

        """

        cursor = self.connection.cursor()

        # The OID integers associated with the geometry type may

        # be different across versions; hence, this is why we have

        # to query the PostgreSQL pg_type table corresponding to the

        # PostGIS custom data types.

        oid_sql = 'SELECT "oid" FROM "pg_type" WHERE "typname" = %s'

        try:

            cursor.execute(oid_sql, ('geometry',))

            GEOM_TYPE = cursor.fetchone()[0]

            postgis_types = { GEOM_TYPE : 'GeometryField' }

            if self.connection.ops.geography:

                cursor.execute(oid_sql, ('geography',))

                GEOG_TYPE = cursor.fetchone()[0]

                # The value for the geography type is actually a tuple

                # to pass in the `geography=True` keyword to the field

                # definition.

                postgis_types[GEOG_TYPE] = ('GeometryField', {'geography' : True})

        finally:

            cursor.close()

        return postgis_types

    def get_field_type(self, data_type, description):

        if not self.postgis_types_reverse:

            # If the PostGIS types reverse dictionary is not populated, do so

            # now.  In order to prevent unnecessary requests upon connection

            # intialization, the `data_types_reverse` dictionary is not updated

            # with the PostGIS custom types until introspection is actually

            # performed -- in other words, when this function is called.

            self.postgis_types_reverse = self.get_postgis_types()

            self.data_types_reverse.update(self.postgis_types_reverse)

        return super(PostGISIntrospection, self).get_field_type(data_type, description)

    def get_geometry_type(self, table_name, geo_col):

        """

        The geometry type OID used by PostGIS does not indicate the particular

        type of field that a geometry column is (e.g., whether it's a

        PointField or a PolygonField).  Thus, this routine queries the PostGIS

        metadata tables to determine the geometry type,

        """

        cursor = self.connection.cursor()

        try:

            try:

                # First seeing if this geometry column is in the `geometry_columns`

                cursor.execute('SELECT "coord_dimension", "srid", "type" '

                               'FROM "geometry_columns" '

                               'WHERE "f_table_name"=%s AND "f_geometry_column"=%s',

                               (table_name, geo_col))

                row = cursor.fetchone()

                if not row: raise GeoIntrospectionError

            except GeoIntrospectionError:

                if self.connection.ops.geography:

                    cursor.execute('SELECT "coord_dimension", "srid", "type" '

                                   'FROM "geography_columns" '

                                   'WHERE "f_table_name"=%s AND "f_geography_column"=%s',

                                   (table_name, geo_col))

                    row = cursor.fetchone()

            if not row:

                raise Exception('Could not find a geometry or geography column for "%s"."%s"' %

                                (table_name, geo_col))

            # OGRGeomType does not require GDAL and makes it easy to convert

            # from OGC geom type name to Django field.

            field_type = OGRGeomType(row[2]).django

            # Getting any GeometryField keyword arguments that are not the default.

            dim = row[0]

            srid = row[1]

            field_params = {}

            if srid != 4326:

                field_params['srid'] = srid

            if dim != 2:

                field_params['dim'] = dim

        finally:

            cursor.close()

        return field_type, field_params