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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
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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

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