====================================
``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 -- thus, when SQL statements contain geometries, it is
    easy to consume more memory than is typical.

Example
=======

1. 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]]

2. 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

3. 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` just 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.4 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
