GeoDJango: retrieve last inserted primary key from LayerMapping

Question

I am building an application with GeoDjango and I have the following problem:

I need to read track data from a GPX file and those data should be stored in a model MultiLineStringField field. This should happen in the admin interface, where the user uploads a GPX file

I am trying to achieve this, namely that the data grabbed from the file should be assigned to the MultiLineStringField, while the other fields should get values from the form.

My model is:

class GPXTrack(models.Model):
    nome = models.CharField("Nome", blank = False, max_length = 255)
    slug = models.SlugField("Slug", blank = True)
    # sport natura arte/cultura
    tipo = models.CharField("Tipologia", blank = False, max_length = 2, choices=TIPOLOGIA_CHOICES)
    descrizione = models.TextField("Descrizione", blank = True)

    gpx_file = models.FileField(upload_to = 'uploads/gpx/')
    track = models.MultiLineStringField(blank = True)
    objects = models.GeoManager()
    published = models.BooleanField("Pubblicato")
    rel_files = generic.GenericRelation(MyFiles)
     #publish_on = models.DateTimeField("Pubblicare il", auto_now_add = True)

    created = models.DateTimeField("Created", auto_now_add = True)
    updated = models.DateTimeField("Updated", auto_now = True)

    class Meta:
       #verbose_name = "struttura'"
       #verbose_name_plural = "strutture"
       ordering = ['-created']

    def __str__(self):
        return str(self.nome)

    def __unicode__(self):
        return '%s' % (self.nome)

    def put(self):
      self.slug = sluggy(self.nome)

      key = super(Foresta, self).put()
      # do something after save
      return key

While in the admin.py file I have overwritten the save method as follows:

    from django.contrib.gis import admin
from trails.models import GPXPoint, GPXTrack
from django.contrib.contenttypes import generic
from django.contrib.gis.gdal import DataSource
#from gpx_mapping import GPXMapping
from django.contrib.gis.utils import LayerMapping
from django.template import RequestContext
import tempfile
import os
import pprint

class GPXTrackAdmin(admin.OSMGeoAdmin):
    list_filter = ( 'tipo', 'published')
    search_fields = ['nome']
    list_display = ('nome', 'tipo', 'published', 'gpx_file')
    inlines = [TrackImagesInline, TrackFilesInline]
    prepopulated_fields = {"slug": ("nome",)}

    def save_model(self, request, obj, form, change):
        """When creating a new object, set the creator field.
        """
        if 'gpx_file' in request.FILES:
            # Get
            gpxFile = request.FILES['gpx_file']
            # Save
            targetPath = tempfile.mkstemp()[1]
            destination = open(targetPath, 'wt')
            for chunk in gpxFile.chunks():
                destination.write(chunk)
            destination.close()

            #define fields of interest for LayerMapping
            track_point_mapping = {'timestamp' : 'time',
                                   'point' : 'POINT',
                          }

            track_mapping = {'track' : 'MULTILINESTRING'}

            gpx_file = DataSource(targetPath)
            mytrack = LayerMapping(GPXTrack, gpx_file, track_mapping, layer='tracks')
            mytrack.save()

            #remove the temp file saved
            os.remove(targetPath)
            orig = GPXTrack.objects.get(pk=mytrack.pk)
            #assign the parsed values from LayerMapping to the appropriate Field
            obj.track = orig.track
            obj.save()

As far as I know:

1. LayerMapping cannot be used to update a field but only to save a new one
2. I cannot access a specific field of the LayerMapping object (ie in the code above: mytrack.track) and assign its value to a model field (ie obj.track) in the model_save method
3. I cannot retrieve the primary key of the last saved LayerMapping object (ie in the code above: mytrack.pk) in order to update it with the values passed in the form for the field not mapped in LayerMapping.mapping

What can I do then?!?!

Answer 1

I sorted it out subclassing LayerMapping and adding a method get_values() that instead of saving the retrieved data, returns them for any use or manipulation.The get_values method is a copy of the LayerMapping::save() method that returns the values instead of saving them. I am using django 1.5

import os
from django.contrib.gis.utils import LayerMapping
import sys
class MyMapping(LayerMapping):
    def get_values(self, verbose=False, fid_range=False, step=False,
         progress=False, silent=False, stream=sys.stdout, strict=False):
        """
        Returns the contents from the OGR DataSource Layer 
        according to the mapping dictionary given at initialization.

        Keyword Parameters:
         verbose:
           If set, information will be printed subsequent to each model save
           executed on the database.

         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.

         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.

         progress:
           When this keyword is set, status information will be printed giving
           the number of features processed and sucessfully saved.  By default,
           progress information will pe printed every 1000 features processed,
           however, this default may be overridden by setting this keyword with an
           integer for the desired interval.

         stream:
           Status information will be written to this file handle.  Defaults to
           using `sys.stdout`, but any object with a `write` method is supported.

         silent:
           By default, non-fatal error notifications are printed to stdout, but
           this keyword may be set to disable these notifications.

         strict:
           Execution of the model mapping will cease upon the first error
           encountered.  The default behavior is to attempt to continue.
        """     
            # Getting the default Feature ID range.
        default_range = self.check_fid_range(fid_range)

        # Setting the progress interval, if requested.
        if progress:
            if progress is True or not isinstance(progress, int):
                progress_interval = 1000
            else:
                progress_interval = progress

        # Defining the 'real' save method, utilizing the transaction
        # decorator created during initialization.
        @self.transaction_decorator
        def _get_values(feat_range=default_range, num_feat=0, num_saved=0):
            if feat_range:
                layer_iter = self.layer[feat_range]
            else:
                layer_iter = self.layer

            for feat in layer_iter:
                num_feat += 1
                # Getting the keyword arguments
                try:
                    kwargs = self.feature_kwargs(feat)
                except LayerMapError, msg:
                    # Something borked the validation
                    if strict: raise
                    elif not silent:
                        stream.write('Ignoring Feature ID %s because: %s\n' % (feat.fid, msg))
                else:
                    # Constructing the model using the keyword args
                    is_update = False
                    if self.unique:
                        # If we want unique models on a particular field, handle the
                        # geometry appropriately.
                        try:
                            # Getting the keyword arguments and retrieving
                            # the unique model.
                            u_kwargs = self.unique_kwargs(kwargs)
                            m = self.model.objects.using(self.using).get(**u_kwargs)
                            is_update = True

                            # Getting the geometry (in OGR form), creating
                            # one from the kwargs WKT, adding in additional
                            # geometries, and update the attribute with the
                            # just-updated geometry WKT.
                            geom = getattr(m, self.geom_field).ogr
                            new = OGRGeometry(kwargs[self.geom_field])
                            for g in new: geom.add(g)
                            setattr(m, self.geom_field, geom.wkt)
                        except ObjectDoesNotExist:
                            # No unique model exists yet, create.
                            m = self.model(**kwargs)
                    else:
                        m = self.model(**kwargs)

                    try:
                        # Attempting to save.
                        pippo = kwargs

                        num_saved += 1
                        if verbose: stream.write('%s: %s\n' % (is_update and 'Updated' or 'Saved', m))
                    except SystemExit:
                        raise
                    except Exception, msg:
                        if self.transaction_mode == 'autocommit':
                            # Rolling back the transaction so that other model saves
                            # will work.
                            transaction.rollback_unless_managed()
                        if strict:
                            # Bailing out if the `strict` keyword is set.
                            if not silent:
                                stream.write('Failed to save the feature (id: %s) into the model with the keyword arguments:\n' % feat.fid)
                                stream.write('%s\n' % kwargs)
                            raise
                        elif not silent:
                            stream.write('Failed to save %s:\n %s\nContinuing\n' % (kwargs, msg))

                # Printing progress information, if requested.
                if progress and num_feat % progress_interval == 0:
                    stream.write('Processed %d features, saved %d ...\n' % (num_feat, num_saved))

            # Only used for status output purposes -- incremental saving uses the
            # values returned here.
            return pippo

        nfeat = self.layer.num_feat
        if step and isinstance(step, int) and step < nfeat:
            # Incremental saving is requested at the given interval (step)
            if default_range:
                raise LayerMapError('The `step` keyword may not be used in conjunction with the `fid_range` keyword.')
            beg, num_feat, num_saved = (0, 0, 0)
            indices = range(step, nfeat, step)
            n_i = len(indices)

            for i, end in enumerate(indices):
                # Constructing the slice to use for this step; the last slice is
                # special (e.g, [100:] instead of [90:100]).
                if i + 1 == n_i: step_slice = slice(beg, None)
                else: step_slice = slice(beg, end)

                try:
                    pippo = _get_values(step_slice, num_feat, num_saved)
                    beg = end
                except:
                    stream.write('%s\nFailed to save slice: %s\n' % ('=-' * 20, step_slice))
                    raise
        else:
            # Otherwise, just calling the previously defined _save() function.
            return _get_values()

In a custom save or save_model method you can then use:

        track_mapping = {'nome': 'name',
                         'track' : 'MULTILINESTRING'}

        targetPath = "/my/gpx/file/path.gpx"
        gpx_file = DataSource(targetPath)

        mytrack = MyMapping(GPXTrack, gpx_file, track_mapping, layer='tracks')

        pippo = mytrack.get_values()
        obj.track = pippo['track']