====================================

``LayerMapping`` data import utility

====================================

.. module:: django.contrib.gis.utils.layermapping

    :synopsis: Spatial data import utility for GeoDjango models.

.. currentmodule:: django.contrib.gis.utils

The :class:`LayerMapping` class provides a way to map the contents of

vector spatial data files (e.g. shapefiles) into GeoDjango models.

This utility grew out of the author's personal needs to eliminate

the code repetition that went into pulling geometries and fields out of

a vector layer, converting to another coordinate system (e.g. WGS84), and

then inserting into a GeoDjango model.

.. note::

    Use of :class:`LayerMapping` requires GDAL.

.. warning ::

    GIS data sources, like shapefiles, may be very large.  If you find

    that :class:`LayerMapping` is using too much memory, set

    :setting:`DEBUG` to ``False`` in your settings.  When :setting:`DEBUG`

    is set to ``True``, Django :ref:`automatically logs <faq-see-raw-sql-queries>`

    *every* SQL query -- and when SQL statements contain geometries, this may

    consume more memory than is typical.

Example

=======

#. You need a GDAL-supported data source, like a shapefile (here we're using

   a simple polygon shapefile, ``test_poly.shp``, with three features)::

    >>> from django.contrib.gis.gdal import DataSource

    >>> ds = DataSource('test_poly.shp')

    >>> layer = ds[0]

    >>> print(layer.fields) # Exploring the fields in the layer, we only want the 'str' field.

    ['float', 'int', 'str']

    >>> print(len(layer)) # getting the number of features in the layer (should be 3)

    3

    >>> print(layer.geom_type) # Should be 'Polygon'

    Polygon

    >>> print(layer.srs) # WGS84 in WKT

    GEOGCS["GCS_WGS_1984",

        DATUM["WGS_1984",

            SPHEROID["WGS_1984",6378137,298.257223563]],

        PRIMEM["Greenwich",0],

        UNIT["Degree",0.017453292519943295]]

#. Now we define our corresponding Django model (make sure to use :djadmin:`migrate`)::

    from django.contrib.gis.db import models

    class TestGeo(models.Model):

        name = models.CharField(max_length=25) # corresponds to the 'str' field

        poly = models.PolygonField(srid=4269) # we want our model in a different SRID

        def __str__(self):

            return 'Name: %s' % self.name

#. Use :class:`LayerMapping` to extract all the features and place them in the

   database::

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

    >>> from geoapp.models import TestGeo

    >>> mapping = {'name' : 'str', # The 'name' model field maps to the 'str' layer field.

                   'poly' : 'POLYGON', # For geometry fields use OGC name.

                   } # The mapping is a dictionary

    >>> lm = LayerMapping(TestGeo, 'test_poly.shp', mapping)

    >>> lm.save(verbose=True) # Save the layermap, imports the data.

    Saved: Name: 1

    Saved: Name: 2

    Saved: Name: 3

Here, :class:`LayerMapping` transformed the three geometries from the shapefile

in their original spatial reference system (WGS84) to the spatial reference

system of the GeoDjango model (NAD83).  If no spatial reference system is

defined for the layer, use the ``source_srs`` keyword with a

:class:`~django.contrib.gis.gdal.SpatialReference` object to specify one.

``LayerMapping`` API

====================

.. class:: LayerMapping(model, data_source, mapping, layer=0, source_srs=None, encoding=None, transaction_mode='commit_on_success', transform=True, unique=True, using='default')

The following are the arguments and keywords that may be used during

instantiation of ``LayerMapping`` objects.

=================  =========================================================

Argument           Description

=================  =========================================================

``model``          The geographic model, *not* an instance.

``data_source``    The path to the OGR-supported data source file

                   (e.g., a shapefile).  Also accepts

                   :class:`django.contrib.gis.gdal.DataSource` instances.

``mapping``        A dictionary: keys are strings corresponding to

                   the model field, and values correspond to

                   string field names for the OGR feature, or if the

                   model field is a geographic then it should

                   correspond to the OGR geometry type,

                   e.g., ``'POINT'``, ``'LINESTRING'``, ``'POLYGON'``.

=================  =========================================================

=====================  =====================================================

Keyword Arguments

=====================  =====================================================

``layer``              The index of the layer to use from the Data Source

                       (defaults to 0)

``source_srs``         Use this to specify the source SRS manually (for

                       example, some shapefiles don't come with a ``'.prj'``

                       file).  An integer SRID, WKT or PROJ strings, and

                       :class:`django.contrib.gis.gdal.SpatialReference`

                       objects are accepted.

``encoding``           Specifies the character set encoding of the strings

                       in the OGR data source.  For example, ``'latin-1'``,

                       ``'utf-8'``, and ``'cp437'`` are all valid encoding

                       parameters.

``transaction_mode``   May be ``'commit_on_success'`` (default) or

                       ``'autocommit'``.

``transform``          Setting this to False will disable coordinate

                       transformations.  In other words, geometries will

                       be inserted into the database unmodified from their

                       original state in the data source.

``unique``             Setting this to the name, or a tuple of names,

                       from the given  model will create models unique

                       only to the given name(s). Geometries from

                       each feature will be added into the collection

                       associated with the unique model.  Forces

                       the transaction mode to be ``'autocommit'``.

``using``              Sets the database to use when importing spatial data.

                       Default is ``'default'``.

=====================  =====================================================

``save()`` Keyword Arguments

----------------------------

.. method:: LayerMapping.save(verbose=False, fid_range=False, step=False, progress=False, silent=False, stream=sys.stdout, strict=False)

The ``save()`` method also accepts keywords.  These keywords are

used for controlling output logging, error handling, and for importing

specific feature ranges.

===========================  =================================================

Save Keyword Arguments       Description

===========================  =================================================

``fid_range``                May be set with a slice or tuple of

                             (begin, end) feature ID's to map from

                             the data source.  In other words, this

                             keyword enables the user to selectively

                             import a subset range of features in the

                             geographic data source.

``progress``                 When this keyword is set, status information

                             will be printed giving the number of features

                             processed and successfully saved.  By default,

                             progress information will be printed every 1000

                             features processed, however, this default may

                             be overridden by setting this keyword with an

                             integer for the desired interval.

``silent``                   By default, non-fatal error notifications are

                             printed to ``sys.stdout``, but this keyword may

                             be set to disable these notifications.

``step``                     If set with an integer, transactions will

                             occur at every step interval. For example, if

                             ``step=1000``, a commit would occur after the

                             1,000th feature, the 2,000th feature etc.

``stream``                   Status information will be written to this file

                             handle.  Defaults to using ``sys.stdout``, but

                             any object with a ``write`` method is supported.

``strict``                   Execution of the model mapping will cease upon

                             the first error encountered.  The default value

                             (``False``)

                             behavior is to attempt to continue.

``verbose``                  If set, information will be printed

                             subsequent to each model save

                             executed on the database.

===========================  =================================================

Troubleshooting

===============

Running out of memory

---------------------

As noted in the warning at the top of this section, Django stores all SQL

queries when ``DEBUG=True``.  Set ``DEBUG=False`` in your settings, and this

should stop excessive memory use when running ``LayerMapping`` scripts.

MySQL: ``max_allowed_packet`` error

-----------------------------------

If you encounter the following error when using ``LayerMapping`` and MySQL::

    OperationalError: (1153, "Got a packet bigger than 'max_allowed_packet' bytes")

Then the solution is to increase the value of the ``max_allowed_packet``

setting in your MySQL configuration.  For example, the default value may

be something low like one megabyte -- the setting may be modified in MySQL's

configuration file (``my.cnf``) in the ``[mysqld]`` section::

    max_allowed_packet = 10M