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POProgrammatically specifying Django model attributes
 

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    <p>I would like to add attributes to a Django models programmatically. At class creation time (the time of the definition of the model class). The model is not going to change after that in run time. For instance, lets say I want to define a <code>Car</code> model class and want to add one <code>price</code> attribute (database column) per currency, given a list of currencies. (This list of currencies should be considered a constant that won't change runtime. I don't want a related model for these prices.)</p> <p>What would be the best way to do this?</p> <p>I had an approach that I thought would work, but it didn't exactly. This is how I tried doing it, using the car example above:</p> <pre><code>from django.db import models class Car(models.Model): name = models.CharField(max_length=50) currencies = ['EUR', 'USD'] for currency in currencies: Car.add_to_class('price_%s' % currency.lower(), models.IntegerField()) </code></pre> <p>This does seem to work pretty well at first sight:</p> <pre><code>$ ./manage.py syncdb Creating table shop_car $ ./manage.py dbshell shop=# \d shop_car Table "public.shop_car" Column | Type | Modifiers -----------+-----------------------+------------------------------------------------------- id | integer | not null default nextval('shop_car_id_seq'::regclass) name | character varying(50) | not null price_eur | integer | not null price_usd | integer | not null Indexes: "shop_car_pkey" PRIMARY KEY, btree (id) </code></pre> <p>But when I try to create a new Car, it doesn't really work anymore:</p> <pre><code>&gt;&gt;&gt; from shop.models import Car &gt;&gt;&gt; mycar = Car(name='VW Jetta', price_eur=100, price_usd=130) &gt;&gt;&gt; mycar &lt;Car: Car object&gt; &gt;&gt;&gt; mycar.save() Traceback (most recent call last): File "&lt;console&gt;", line 1, in &lt;module&gt; File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/base.py", line 410, in save self.save_base(force_insert=force_insert, force_update=force_update) File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/base.py", line 495, in save_base result = manager._insert(values, return_id=update_pk) File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/manager.py", line 177, in _insert return insert_query(self.model, values, **kwargs) File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/query.py", line 1087, in insert_query return query.execute_sql(return_id) File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/sql/subqueries.py", line 320, in execute_sql cursor = super(InsertQuery, self).execute_sql(None) File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/sql/query.py", line 2369, in execute_sql cursor.execute(sql, params) File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/backends/util.py", line 19, in execute return self.cursor.execute(sql, params) ProgrammingError: column "price_eur" specified more than once LINE 1: ...NTO "shop_car" ("name", "price_eur", "price_usd", "price_eur... ^ </code></pre> <p>Apparently, though, my code seem to run several times, causing the "price_eur" attribute to be added several times.</p> <p>Comment: Initially I used the wording "at run time" ("I would like to add attributes to a Django models programmatically, at run time."). This wording wasn't the best. What I really want to was to add those fields at "model definition time" or "class creation time".</p>
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    1. COOn a related note - I know it is ugly to add fields programatically in this way. A more "beautiful" way would be to use a related model. But that is not the question here, as added database complexity and reduced performance in this case is simply not worth it.
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    2. CO"reduced performance"? Have you measured it? You'll be hard-pressed to see the effect of this kind of exact-match relational join.
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    3. CO@S.Lott: Yes. I haven't measured exactly this particular case, but I have examples with large datasets (at one side of the join) where additional SQL joins have simply been the big performance issue in certain queries. You could off course be right that in this particular case it might not have that big effect.
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