# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.contrib.gis.db.models.fields

    from django.contrib.gis import forms
    from django.contrib.gis.db.models.lookups import (
        RasterBandTransform, gis_lookups,
    )
    from django.contrib.gis.db.models.proxy import SpatialProxy
    from django.contrib.gis.gdal import HAS_GDAL
    from django.contrib.gis.gdal.error import GDALException
    from django.contrib.gis.geometry.backend import Geometry, GeometryException
    from django.core.exceptions import ImproperlyConfigured
    from django.db.models.expressions import Expression
    from django.db.models.fields import Field
    from django.utils import six
    from django.utils.translation import ugettext_lazy as _

    # Local cache of the spatial_ref_sys table, which holds SRID data for each
    # spatial database alias. This cache exists so that the database isn't queried
    # for SRID info each time a distance query is constructed.
    _srid_cache = {}

    def get_srid_info(srid, connection):
        """
        Returns the units, unit name, and spheroid WKT associated with the
        given SRID from the `spatial_ref_sys` (or equivalent) spatial database
        table for the given database connection.  These results are cached.
        """
        global _srid_cache

        try:
            # The SpatialRefSys model for the spatial backend.
            SpatialRefSys = connection.ops.spatial_ref_sys()
        except NotImplementedError:
            # No `spatial_ref_sys` table in spatial backend (e.g., MySQL).
            return None, None, None

        if connection.alias not in _srid_cache:
            # Initialize SRID dictionary for database if it doesn't exist.
            _srid_cache[connection.alias] = {}

        if srid not in _srid_cache[connection.alias]:
            # Use `SpatialRefSys` model to query for spatial reference info.
            sr = SpatialRefSys.objects.using(connection.alias).get(srid=srid)
            units, units_name = sr.units
            spheroid = SpatialRefSys.get_spheroid(sr.wkt)
            _srid_cache[connection.alias][srid] = (units, units_name, spheroid)

        return _srid_cache[connection.alias][srid]

    class GeoSelectFormatMixin(object):
        def select_format(self, compiler, sql, params):
            """
            Returns the selection format string, depending on the requirements
            of the spatial backend.  For example, Oracle and MySQL require custom
            selection formats in order to retrieve geometries in OGC WKT. For all
            other fields a simple '%s' format string is returned.
            """
            connection = compiler.connection
            srid = compiler.query.get_context('transformed_srid')
            if srid:
                sel_fmt = '%s(%%s, %s)' % (connection.ops.transform, srid)
            else:
                sel_fmt = '%s'
            if connection.ops.select:
                # This allows operations to be done on fields in the SELECT,
                # overriding their values -- used by the Oracle and MySQL
                # spatial backends to get database values as WKT, and by the
                # `transform` method.
                sel_fmt = connection.ops.select % sel_fmt
            return sel_fmt % sql, params

    class BaseSpatialField(Field):
        """
        The Base GIS Field.

        It's used as a base class for GeometryField and RasterField. Defines
        properties that are common to all GIS fields such as the characteristics
        of the spatial reference system of the field.
        """
        description = _("The base GIS field.")
        empty_strings_allowed = False
        # Geodetic units.
        geodetic_units = ('decimal degree', 'degree')

        def __init__(self, verbose_name=None, srid=4326, spatial_index=True, **kwargs):
            """
            The initialization function for base spatial fields. Takes the following
            as keyword arguments:

            srid:
             The spatial reference system identifier, an OGC standard.
             Defaults to 4326 (WGS84).

            spatial_index:
             Indicates whether to create a spatial index.  Defaults to True.
             Set this instead of 'db_index' for geographic fields since index
             creation is different for geometry columns.
            """

            # Setting the index flag with the value of the `spatial_index` keyword.
            self.spatial_index = spatial_index

            # Setting the SRID and getting the units.  Unit information must be
            # easily available in the field instance for distance queries.
            self.srid = srid

            # Setting the verbose_name keyword argument with the positional
            # first parameter, so this works like normal fields.
            kwargs['verbose_name'] = verbose_name

            super(BaseSpatialField, self).__init__(**kwargs)

        def deconstruct(self):
            name, path, args, kwargs = super(BaseSpatialField, self).deconstruct()
            # Always include SRID for less fragility; include spatial index if it's
            # not the default value.
            kwargs['srid'] = self.srid
            if self.spatial_index is not True:
                kwargs['spatial_index'] = self.spatial_index
            return name, path, args, kwargs

        def db_type(self, connection):
            return connection.ops.geo_db_type(self)

        # The following functions are used to get the units, their name, and
        # the spheroid corresponding to the SRID of the BaseSpatialField.
        def _get_srid_info(self, connection):
            # Get attributes from `get_srid_info`.
            self._units, self._units_name, self._spheroid = get_srid_info(self.srid, connection)

        def spheroid(self, connection):
            if not hasattr(self, '_spheroid'):
                self._get_srid_info(connection)
            return self._spheroid

        def units(self, connection):
            if not hasattr(self, '_units'):
                self._get_srid_info(connection)
            return self._units

        def units_name(self, connection):
            if not hasattr(self, '_units_name'):
                self._get_srid_info(connection)
            return self._units_name

        def geodetic(self, connection):
            """
            Returns true if this field's SRID corresponds with a coordinate
            system that uses non-projected units (e.g., latitude/longitude).
            """
            units_name = self.units_name(connection)
            # Some backends like MySQL cannot determine units name. In that case,
            # test if srid is 4326 (WGS84), even if this is over-simplification.
            return units_name.lower() in self.geodetic_units if units_name else self.srid == 4326

        def get_placeholder(self, value, compiler, connection):
            """
            Returns the placeholder for the spatial column for the
            given value.
            """
            return connection.ops.get_geom_placeholder(self, value, compiler)

        def get_srid(self, obj):
            """
            Return the default SRID for the given geometry or raster, taking into
            account the SRID set for the field. For example, if the input geometry
            or raster doesn't have an SRID, then the SRID of the field will be
            returned.
            """
            srid = obj.srid  # SRID of given geometry.
            if srid is None or self.srid == -1 or (srid == -1 and self.srid != -1):
                return self.srid
            else:
                return srid

        def get_db_prep_save(self, value, connection):
            """
            Prepare the value for saving in the database.
            """
            if not value:
                return None
            else:
                return connection.ops.Adapter(self.get_prep_value(value))

        def get_raster_prep_value(self, value, is_candidate):
            """
            Return a GDALRaster if conversion is successful, otherwise return None.
            """
            from django.contrib.gis.gdal import GDALRaster

            if isinstance(value, GDALRaster):
                return value
            elif is_candidate:
                try:
                    return GDALRaster(value)
                except GDALException:
                    pass
            elif isinstance(value, dict):
                try:
                    return GDALRaster(value)
                except GDALException:
                    raise ValueError("Couldn't create spatial object from lookup value '%s'." % value)

        def get_prep_value(self, value):
            """
            Spatial lookup values are either a parameter that is (or may be
            converted to) a geometry or raster, or a sequence of lookup values
            that begins with a geometry or raster. This routine sets up the
            geometry or raster value properly and preserves any other lookup
            parameters.
            """
            value = super(BaseSpatialField, self).get_prep_value(value)

            # For IsValid lookups, boolean values are allowed.
            if isinstance(value, (Expression, bool)):
                return value
            elif isinstance(value, (tuple, list)):
                obj = value[0]
                seq_value = True
            else:
                obj = value
                seq_value = False

            # When the input is not a geometry or raster, attempt to construct one
            # from the given string input.
            if isinstance(obj, Geometry):
                pass
            else:
                # Check if input is a candidate for conversion to raster or geometry.
                is_candidate = isinstance(obj, (bytes, six.string_types)) or hasattr(obj, '__geo_interface__')
                # With GDAL installed, try to convert the input to raster.
                raster = False
                if HAS_GDAL:
                    raster = self.get_raster_prep_value(obj, is_candidate)

                if raster:
                    obj = raster
                elif is_candidate:
                    try:
                        obj = Geometry(obj)
                    except (GeometryException, GDALException):
                        raise ValueError("Couldn't create spatial object from lookup value '%s'." % obj)
                else:
                    raise ValueError('Cannot use object with type %s for a spatial lookup parameter.' % type(obj).__name__)

            # Assigning the SRID value.
            obj.srid = self.get_srid(obj)

            if seq_value:
                lookup_val = [obj]
                lookup_val.extend(value[1:])
                return tuple(lookup_val)
            else:
                return obj

    for klass in gis_lookups.values():
        BaseSpatialField.register_lookup(klass)

    class GeometryField(GeoSelectFormatMixin, BaseSpatialField):
        """
        The base Geometry field -- maps to the OpenGIS Specification Geometry type.
        """
        description = _("The base Geometry field -- maps to the OpenGIS Specification Geometry type.")
        form_class = forms.GeometryField
        # The OpenGIS Geometry name.
        geom_type = 'GEOMETRY'

        def __init__(self, verbose_name=None, dim=2, geography=False, **kwargs):
            """
            The initialization function for geometry fields. In addition to the
            parameters from BaseSpatialField, it takes the following as keyword
            arguments:

            dim:
             The number of dimensions for this geometry.  Defaults to 2.

            extent:
             Customize the extent, in a 4-tuple of WGS 84 coordinates, for the
             geometry field entry in the `USER_SDO_GEOM_METADATA` table.  Defaults
             to (-180.0, -90.0, 180.0, 90.0).

            tolerance:
             Define the tolerance, in meters, to use for the geometry field
             entry in the `USER_SDO_GEOM_METADATA` table.  Defaults to 0.05.
            """
            # Setting the dimension of the geometry field.
            self.dim = dim

            # Is this a geography rather than a geometry column?
            self.geography = geography

            # Oracle-specific private attributes for creating the entry in
            # `USER_SDO_GEOM_METADATA`
            self._extent = kwargs.pop('extent', (-180.0, -90.0, 180.0, 90.0))
            self._tolerance = kwargs.pop('tolerance', 0.05)

            super(GeometryField, self).__init__(verbose_name=verbose_name, **kwargs)

        def deconstruct(self):
            name, path, args, kwargs = super(GeometryField, self).deconstruct()
            # Include kwargs if they're not the default values.
            if self.dim != 2:
                kwargs['dim'] = self.dim
            if self.geography is not False:
                kwargs['geography'] = self.geography
            return name, path, args, kwargs

        # ### Routines specific to GeometryField ###
        def get_distance(self, value, lookup_type, connection):
            """
            Returns a distance number in units of the field.  For example, if
            `D(km=1)` was passed in and the units of the field were in meters,
            then 1000 would be returned.
            """
            return connection.ops.get_distance(self, value, lookup_type)

        def from_db_value(self, value, expression, connection, context):
            if value:
                if not isinstance(value, Geometry):
                    value = Geometry(value)
                srid = value.srid
                if not srid and self.srid != -1:
                    value.srid = self.srid
            return value

        # ### Routines overloaded from Field ###
        def contribute_to_class(self, cls, name, **kwargs):
            super(GeometryField, self).contribute_to_class(cls, name, **kwargs)

            # Setup for lazy-instantiated Geometry object.
            setattr(cls, self.attname, SpatialProxy(Geometry, self))

        def formfield(self, **kwargs):
            defaults = {'form_class': self.form_class,
                        'geom_type': self.geom_type,
                        'srid': self.srid,
                        }
            defaults.update(kwargs)
            if (self.dim > 2 and 'widget' not in kwargs and
                    not getattr(defaults['form_class'].widget, 'supports_3d', False)):
                defaults['widget'] = forms.Textarea
            return super(GeometryField, self).formfield(**defaults)

        def _get_db_prep_lookup(self, lookup_type, value, connection):
            """
            Prepare for the database lookup, and return any spatial parameters
            necessary for the query.  This includes wrapping any geometry
            parameters with a backend-specific adapter and formatting any distance
            parameters into the correct units for the coordinate system of the
            field.

            Only used by the deprecated GeoQuerySet and to be
            RemovedInDjango20Warning.
            """
            # Populating the parameters list, and wrapping the Geometry
            # with the Adapter of the spatial backend.
            if isinstance(value, (tuple, list)):
                params = [connection.ops.Adapter(value[0])]
                # Getting the distance parameter in the units of the field.
                params += self.get_distance(value[1:], lookup_type, connection)
            else:
                params = [connection.ops.Adapter(value)]
            return params

    # The OpenGIS Geometry Type Fields
    class PointField(GeometryField):
        geom_type = 'POINT'
        form_class = forms.PointField
        description = _("Point")

    class LineStringField(GeometryField):
        geom_type = 'LINESTRING'
        form_class = forms.LineStringField
        description = _("Line string")

    class PolygonField(GeometryField):
        geom_type = 'POLYGON'
        form_class = forms.PolygonField
        description = _("Polygon")

    class MultiPointField(GeometryField):
        geom_type = 'MULTIPOINT'
        form_class = forms.MultiPointField
        description = _("Multi-point")

    class MultiLineStringField(GeometryField):
        geom_type = 'MULTILINESTRING'
        form_class = forms.MultiLineStringField
        description = _("Multi-line string")

    class MultiPolygonField(GeometryField):
        geom_type = 'MULTIPOLYGON'
        form_class = forms.MultiPolygonField
        description = _("Multi polygon")

    class GeometryCollectionField(GeometryField):
        geom_type = 'GEOMETRYCOLLECTION'
        form_class = forms.GeometryCollectionField
        description = _("Geometry collection")

    class ExtentField(GeoSelectFormatMixin, Field):
        "Used as a return value from an extent aggregate"

        description = _("Extent Aggregate Field")

        def get_internal_type(self):
            return "ExtentField"

    class RasterField(BaseSpatialField):
        """
        Raster field for GeoDjango -- evaluates into GDALRaster objects.
        """

        description = _("Raster Field")
        geom_type = 'RASTER'
        geography = False

        def __init__(self, *args, **kwargs):
            if not HAS_GDAL:
                raise ImproperlyConfigured('RasterField requires GDAL.')
            super(RasterField, self).__init__(*args, **kwargs)

        def _check_connection(self, connection):
            # Make sure raster fields are used only on backends with raster support.
            if not connection.features.gis_enabled or not connection.features.supports_raster:
                raise ImproperlyConfigured('Raster fields require backends with raster support.')

        def db_type(self, connection):
            self._check_connection(connection)
            return super(RasterField, self).db_type(connection)

        def from_db_value(self, value, expression, connection, context):
            return connection.ops.parse_raster(value)

        def get_db_prep_value(self, value, connection, prepared=False):
            self._check_connection(connection)
            # Prepare raster for writing to database.
            if not prepared:
                value = connection.ops.deconstruct_raster(value)
            return super(RasterField, self).get_db_prep_value(value, connection, prepared)

        def contribute_to_class(self, cls, name, **kwargs):
            super(RasterField, self).contribute_to_class(cls, name, **kwargs)
            # Importing GDALRaster raises an exception on systems without gdal.
            from django.contrib.gis.gdal import GDALRaster
            # Setup for lazy-instantiated Raster object. For large querysets, the
            # instantiation of all GDALRasters can potentially be expensive. This
            # delays the instantiation of the objects to the moment of evaluation
            # of the raster attribute.
            setattr(cls, self.attname, SpatialProxy(GDALRaster, self))

        def get_transform(self, name):
            try:
                band_index = int(name)
                return type(
                    'SpecificRasterBandTransform',
                    (RasterBandTransform, ),
                    {'band_index': band_index}
                )
            except ValueError:
                pass
            return super(RasterField, self).get_transform(name)