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Edited docs/db-api.txt changes from [5700]
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@ -125,27 +125,31 @@ When you save an object, Django performs the following steps:
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1. **Emit a ``pre_save`` signal.** This provides a notification that
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an object is about to be saved. You can register a listener that
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will be invoked whenever this signal is emitted.
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will be invoked whenever this signal is emitted. (These signals are
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not yet documented.)
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2. **Pre-process the data.** Each field on the object is asked to
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perform any automated data modification that the field may need
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to perform.
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Most fields do *no* pre-processing - the field data is kept as-is.
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Most fields do *no* pre-processing -- the field data is kept as-is.
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Pre-processing is only used on fields that have special behavior.
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For example, if your model has a ``DateField`` with ``auto_now=True``,
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the pre-save phase will alter the data in the object to ensure that
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the date field contains the current date stamp.
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the date field contains the current date stamp. (Our documentation
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doesn't yet include a list of all the fields with this "special
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behavior.")
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3. **Prepare the data for the database.** Each field is asked to provide
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their current value in a datatype that can be written to the database.
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its current value in a data type that can be written to the database.
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Again, most fields require *no* data preparation. Simple data types,
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such as integers and strings, are 'ready to write' as a Python object.
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However, more complex data types often require some modification. For
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example, ``DateFields`` use a Python ``datetime`` object to store data.
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Databases don't store ``datetime`` objects, so the field value
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must be converted into an ISO compliant date string for insertion
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Most fields require *no* data preparation. Simple data types, such as
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integers and strings, are 'ready to write' as a Python object. However,
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more complex data types often require some modification.
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For example, ``DateFields`` use a Python ``datetime`` object to store
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data. Databases don't store ``datetime`` objects, so the field value
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must be converted into an ISO-compliant date string for insertion
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into the database.
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4. **Insert the data into the database.** The pre-processed, prepared
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@ -154,18 +158,18 @@ When you save an object, Django performs the following steps:
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5. **Emit a ``post_save`` signal.** As with the ``pre_save`` signal, this
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is used to provide notification that an object has been successfully
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saved.
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saved. (These signals are not yet documented.)
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Raw Saves
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~~~~~~~~~
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**New in Django development version**
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The pre-processing step performed by Django is extremely useful for
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implementing special field behavior (such as the ``auto_now`` feature of
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``DateField``), but it does modify the data stored in a field. This can cause
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problems if you are relying upon the data you provide being used as-is. For
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example, if you are setting up conditions for a test, you will want the test
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The pre-processing step (#2 in the previous section) is useful, but it modifies
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the data stored in a field. This can cause problems if you're relying upon the
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data you provide being used as-is.
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For example, if you're setting up conditions for a test, you'll want the test
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conditions to be repeatable. If pre-processing is performed, the data used
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to specify test conditions may be modified, changing the conditions for the
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test each time the test is run.
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@ -182,10 +186,10 @@ insertion, and post-save signal) are performed as normal.
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.. admonition:: When to use a raw save
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Generally speaking, you shouldn't need use use a raw save. Disabling field
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Generally speaking, you shouldn't need to use a raw save. Disabling field
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pre-processing is an extraordinary measure that should only be required
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in extraordinary circumstances (such as setting up reliable test
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conditions).
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in extraordinary circumstances, such as setting up reliable test
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conditions.
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Saving changes to objects
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=========================
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