Skip to content


Subversion checkout URL

You can clone with
Download ZIP
Browse files

Clarified the documentation on the steps that happen during a save, a…

…nd how raw save affects those steps.

git-svn-id: bcc190cf-cafb-0310-a4f2-bffc1f526a37
  • Loading branch information...
commit eff675b91eac5cf1fbfc730184bb582ce8175693 1 parent 55de110
@freakboy3742 freakboy3742 authored
Showing with 63 additions and 14 deletions.
  1. +63 −14 docs/db-api.txt
77 docs/db-api.txt
@@ -118,25 +118,74 @@ happens.
Explicitly specifying auto-primary-key values is mostly useful for bulk-saving
objects, when you're confident you won't have primary-key collision.
-Raw saves
+What happens when you save?
-**New in Django development version**
+When you save an object, Django performs the following steps:
+ 1. **Emit a ``pre_save`` signal.** This provides a notification that
+ an object is about to be saved. You can register a listener that
+ will be invoked whenever this signal is emitted.
+ 2. **Pre-process the data.** Each field on the object is asked to
+ perform any automated data modification that the field may need
+ to perform.
+ Most fields do *no* pre-processing - the field data is kept as-is.
+ Pre-processing is only used on fields that have special behavior.
+ For example, if your model has a ``DateField`` with ``auto_now=True``,
+ the pre-save phase will alter the data in the object to ensure that
+ the date field contains the current date stamp.
+ 3. **Prepare the data for the database.** Each field is asked to provide
+ their current value in a datatype that can be written to the database.
+ Again, most fields require *no* data preparation. Simple data types,
+ such as integers and strings, are 'ready to write' as a Python object.
+ However, more complex data types often require some modification. For
+ example, ``DateFields`` use a Python ``datetime`` object to store data.
+ Databases don't store ``datetime`` objects, so the field value
+ must be converted into an ISO compliant date string for insertion
+ into the database.
+ 4. **Insert the data into the database.** The pre-processed, prepared
+ data is then composed into an SQL statement for insertion into the
+ database.
-When you save an Django object, some pre-processing might occur on the the
-object's data before it's saved to the database. For example, if your model has
-a ``DateField`` with ``auto_now=True`` set, the pre-save phase will alter the
-data in the object to ensure that the date field contains the current date
+ 5. **Emit a ``post_save`` signal.** As with the ``pre_save`` signal, this
+ is used to provide notification that an object has been successfully
+ saved.
-Although these automated changes can be useful, sometimes you just want to save
-the data as-is. In these cases, you can invoke a **raw save** by passing
+Raw Saves
+**New in Django development version**
+The pre-processing step performed by Django is extremely useful for
+implementing special field behavior (such as the ``auto_now`` feature of
+``DateField``), but it does modify the data stored in a field. This can cause
+problems if you are relying upon the data you provide being used as-is. For
+example, if you are setting up conditions for a test, you will want the test
+conditions to be repeatable. If pre-processing is performed, the data used
+to specify test conditions may be modified, changing the conditions for the
+test each time the test is run.
+In cases such as this, you need to prevent pre-processing from being performed
+when you save an object. To do this, you can invoke a **raw save** by passing
``raw=True`` as an argument to the ``save()`` method::
- # Saves object, but does no pre-processing
+ # Save object, but do no pre-processing
-A raw save saves all the data in your object, but performs no pre-save processing
-on the data in the object.
+A raw save skips the usual data pre-processing that is performed during the
+save. All other steps in the save (pre-save signal, data preparation, data
+insertion, and post-save signal) are performed as normal.
+.. admonition:: When to use a raw save
+ Generally speaking, you shouldn't need use use a raw save. Disabling field
+ pre-processing is an extraordinary measure that should only be required
+ in extraordinary circumstances (such as setting up reliable test
+ conditions).
Saving changes to objects
@@ -160,7 +209,7 @@ object of the right type to the field in question::
joe = Author.objects.create(name="Joe") = joe
Django will complain if you try to assign an object of the wrong type.
How Django knows to UPDATE vs. INSERT
Please sign in to comment.
Something went wrong with that request. Please try again.