GeoDjango LayerMapping & Foreign Key

Question

I am trying to import my KML file into a model using GeoDjango's LayerMapping functionality. I've run tests and had no issues when doing regular imports. However, I recently added a foreign key to my model. My model is called PlaceMark and it now has a FK to a model called Layer. I would like to either

1. override the import and manually set the value of the foreign key field or
2. update my KML file to contain a new element that connects the PlaceMark to the layer via either the pk or name field of Layer.

Here is how I am testing from the shell and the relevant error:

>>>from locator import load
>>>load.run()
...
TypeError: ForeignKey mapping must be of dictionary type.
....

Here is my load.py file:

import os
from django.contrib.gis.utils import LayerMapping
from models import PlaceMark

placemark_mapping = {
    'name' : 'Name',
    'description' : 'Description',
    # This line below is the one that is suspect #
    'layer': 'Layer',
    'geom' : 'POINT25D',
}

placemark_kml = os.path.abspath(os.path.join(os.path.dirname(__file__), 'data/claim.kml'))

def run(verbose=True):
    lm = LayerMapping(PlaceMark, placemark_kml, placemark_mapping,
                      transform=False, encoding='iso-8859-1')

lm.save(strict=True, verbose=verbose)

KML File:

<?xml version="1.0" encoding="Windows-1252"?>
<kml xmlns="http://earth.google.com/kml/2.1">
<Folder>
  <description><![CDATA[TankSafe_Claims]]></description>
  <Placemark>
    <name><![CDATA[G2184729A]]></name>
    <description><![CDATA[<br><br><br>
    <table border="1" padding="0">
    <tr><td>Policy_Number</td><td>53645645</td></tr>
    <tr><td>Claim_Number</td><td>2342342234</td></tr>
    <tr><td>Policy_Type</td><td>TSP</td></tr>
    <tr><td>Name</td><td>Al's Total</td></tr>
    <tr><td>Street_Address</td><td>555 109th Avenue</td></tr>
    <tr><td>City</td><td>Pullman</td></tr>
    <tr><td>State</td><td>NY</td></tr>
    <tr><td>Zip_Code</td><td>55555</td></tr>
    <tr><td>County</td><td>Allegan</td></tr>
        ]]></description>
    <visibility>1</visibility>
    <open>0</open>
    <Point>
      <extrude>1</extrude>
      <altitudeMode>relativeToGround</altitudeMode>
      <coordinates>-86.092641,42.483953,0</coordinates>
    </Point>
    <!--- ***Should I add the line below?*** -->
    <Layer><name>claims</name></Layer>
  </Placemark>
</Folder>
</kml>

My goal is to just get all the PlaceMarks imported with references to the relevant layer. Any ideas?

Thanks! Larry

Answer 1

layer_mapping = {
    'fk': {'nm_field': 'NAME'}, # foreign key field
    'this_field': 'THIS',
    'that_field': 'THAT',
    'geom': 'POLYGON',
}

the error you're receiving that the Foreign Key field should be a dictionary is basically requesting an additional mapping to the model which the foreign key relates.

in the above snippet:

• 'fk' is the foreign key field name from the model the data is being loaded into (lets call it 'load model')
• 'nm_field' is the field name from the model the 'load model' has the foreign key relationship to (lets call it 'primary model')
• 'NAME' is the field name from the data being loaded into 'load model' which holds the relationship to 'primary model'

more explicitly, imagine if 'primary model' is a dataset of lakes and they have a field called 'nm_field' that is the lake name as a string.

now imagine, 'load model' is a dataset of points representing all the buoys on all the lakes and has a field name 'fk' that is a ForeignKey to 'primary model' for the assignment of the lake each buoy belongs to.

finally, the data you're loading into 'load model' has a string field called 'NAME' and it contains the pre-populated name of the lake each buoy belongs to. that string name is the relationship tie. it allows the 'load model' to use that name to identify which lake in the 'primary model' it should establish a foreign key with.