Changes between Version 14 and Version 15 of new_meta_api

Timestamp:

Jul 11, 2014, 7:33:27 AM (10 years ago)

Author:

pirosb3

Comment:

--

new_meta_api

@@ -200 +200 @@
     *** FieldDoesNotExist: User has no field named 'does_not_exist'
 }}}
+
+
+=== The Decision Process
+
+Since I started my Summer of Code project, this API has gone through several designs, and has now finalised onto the one shown above.
+The API has gone through many transformations. Each decision has gone through my mentor, with whom I have weekly meetings (Russell).
+
+==== Using bitfields as flags
+
+get_field and get_fields were originally designed to work with bits. The main choice for this decision was because there were many options and,
+in order to avoid providing multiple flags, it would be better to provide bits.
+The original API for bits is:
+
+{{{
+    DATA = 0b00001
+    M2M = 0b00010
+    RELATED_OBJECTS = 0b00100
+    RELATED_M2M = 0b01000
+    VIRTUAL = 0b10000
+
+    # Aggregates
+    NON_RELATED_FIELDS = DATA | M2M | VIRTUAL
+    ALL = RELATED_M2M | RELATED_OBJECTS | M2M | DATA | VIRTUAL
+
+    NONE = 0b0000
+    LOCAL_ONLY = 0b0001
+    CONCRETE = 0b0010
+    INCLUDE_HIDDEN = 0b0100
+    INCLUDE_PROXY = 0b1000
+
+    def get_fields(types, opts)
+}}}
+
+There are numerous reasons why we backed away from this design:
+1) There is always a need to import flags from models/options, this can bring to circular dependencies
+2) Importing flags all the time can also be a nuinsance
+2) Importing flags is not Pythonic at all
+
+The decision taken was to port 'get_field' and 'get_fields' to flags.
+A port of the old implementation lies here if you are interested: https://github.com/PirosB3/django/blob/soc2014_meta_refactor_upgrade/django/db/models/options.py
+
+==== Removed direct, m2m, model
+In the previous API, it was a common pattern to return model, direct (bool), m2m (bool). I soon realized that not only
+these three paramenters can be easily derived from a field_instance, but there were very few places that actually used some of
+the attributes (there is only 1 place where m2m is used).
+
+The decision taken was to drop direct, m2m, model in the return type and only keep field_instance. All the rest will be derived.
+
+==== Removed all calls "with_model"
+As said previously, it is redundant to include any model as this can be derived.
+
+==== Removed the need of multiple maps
+The previous implementation relied on many different cache maps internally. This is somewhat necessary, but tends to increase bug-risk
+when cache-expiry happens. For this reason, my implementation relies only on 2 cache tables, and I have added a specific function to do
+cache expiry (called _expire_cache) that will wipe out all memory.
+The downsides if this aspect is that we cache a bit more naively (there are less layers of caching) but benchmarks show this does not
+decrease performance.