#14 #15 Serialize EDTFField values into DB

Improve performance of EDTFField when populating model
instances from DB values by storing EDTF data in pickled
format, not as string values that need to be re-parsed
every time they are loaded.

This implementation is naive and could be improved with
a more sophisticated serialization approach for EDTF
fields, though it works for now.

This change also handles the issue in #15 where already-
parsed EDTF field values are re-parsed when an instance
is saved, unless there is a `natural_text_field` value
present to override and reset the EDTF field value.

edtf/fields.py

@@ -1,3 +1,8 @@
+try:
+    import cPickle as pickle
+except:
+    import pickle
+
 from django.db import models
 
 from edtf import parse_edtf, EDTFObject
@@ -22,7 +27,7 @@ def __init__(
         upper_fuzzy_field=None,
         **kwargs
     ):
-        kwargs['max_length'] = 255
+        kwargs['max_length'] = 1000
         self.natural_text_field, self.lower_strict_field, \
         self.upper_strict_field, self.lower_fuzzy_field, \
         self.upper_fuzzy_field = natural_text_field, lower_strict_field, \
@@ -48,6 +53,12 @@ def deconstruct(self):
 
     def from_db_value(self, value, expression, connection, context):
         # Converting values to Python objects
+        if not value:
+            return None
+        try:
+            return pickle.loads(str(value))
+        except:
+            pass
         return parse_edtf(value, fail_silently=True)
 
     def to_python(self, value):
@@ -59,11 +70,16 @@ def to_python(self, value):
 
         return parse_edtf(value, fail_silently=True)
 
+    def get_db_prep_save(self, value, connection):
+        if value:
+            return pickle.dumps(value)
+        return super(EDTFField, self).get_db_prep_save(value, connection)
+
     def get_prep_value(self, value):
         # convert python objects to query values
         value = super(EDTFField, self).get_prep_value(value)
         if isinstance(value, EDTFObject):
-            return unicode(value)
+            return pickle.dumps(value)
         return value
 
     def pre_save(self, instance, add):
@@ -74,19 +90,29 @@ def pre_save(self, instance, add):
         if not self.natural_text_field or self.attname not in instance.__dict__:
             return
 
+        edtf = getattr(instance, self.attname)
+
+        # Update EDTF field based on latest natural text value, if any
         natural_text = getattr(instance, self.natural_text_field)
         if natural_text:
-            n = text_to_edtf(natural_text)
-            setattr(instance, self.attname, n)
+            edtf = text_to_edtf(natural_text)
 
-        e = parse_edtf(getattr(instance, self.attname), fail_silently=True)
-        if e:
+        # TODO If `natural_text_field` becomes cleared the derived EDTF field
+        # value should also be cleared, rather than left at original value?
 
+        # TODO Handle case where EDTF field is set to a string directly, not
+        # via `natural_text_field` (this is a slightly unexpected use-case, but
+        # is a very efficient way to set EDTF values in situations like for API
+        # imports so we probably want to continue to support it?)
+        if edtf and not isinstance(edtf, EDTFObject):
+            edtf = parse_edtf(edtf, fail_silently=True)
+
+        setattr(instance, self.attname, edtf)
+        if edtf:
             # set related date fields on the instance
             for attr in DATE_ATTRS:
                 field_attr = "%s_field" % attr
                 g = getattr(self, field_attr, None)
                 if g:
-                    setattr(instance, g, getattr(e, attr)())
-
-            return unicode(e)
+                    setattr(instance, g, getattr(edtf, attr)())
+        return edtf