mibios/query: Add ChainedQuerySet container/wrapper

The ChainedQuerySet hooks into the QuerySet.iterate() machinery via the
new QuerySet.split_by_fk() method to be used as alternative to a
filter(relation__in=subquery) when the subquery prevents an index scan
(the first application will be data export of ReadAbundance for a
selection of samples.)

This is part 1 of 3 of series of commits to further address slow data
export of very large tables.

mibios/query.py

@@ -1,6 +1,7 @@
 from collections import OrderedDict
 from decimal import Decimal
 from inspect import getgeneratorstate, getgeneratorlocals
+from itertools import chain
 import json
 from logging import getLogger
 from operator import attrgetter, itemgetter
@@ -1043,3 +1044,110 @@ def iterate(self, chunk_size=None, cache=None):
             return ModelIterable(self, chunk_size, cache)
         else:
             return ValuesListIterable(self, chunk_size, cache)
+
+    def split_by_fk(self, fk_field, subquery):
+        return ChainedQuerySet(self, fk_field, subquery)
+
+
+class ChainedQuerySet:
+    """
+    Chain querysets, split along relation to avoid WHERE w/subquery.
+
+    This is a wrapper/container for querysets.  It replaces a Queryset that had
+    filter(relation__in=subquery) applied with a chain of querysets with
+    filter(relation__pk=a_pk) for each object in the subquery.  The resulting
+    objects can with limitations be used like the original queryset.
+
+    It is primarily meant to have iterate() applied further splitting the
+    queries in order to take advantange of indexes and avoiding sequence scan
+    on very large tables..
+    """
+    def __init__(self, queryset, fk_field, subqueryset):
+        model = queryset.model
+        if isinstance(fk_field, str):
+            fk_field = model._meta.get_field(fk_field)
+        else:
+            try:
+                _fk_field = model._meta.get_field(fk_field.name)
+            except FieldDoesNotExist:
+                raise ValueError('field does not belong to model of queryset')
+            else:
+                if _fk_field is not fk_field:
+                    raise ValueError('not a field of model (but name match?)')
+        if not fk_field.many_to_one:
+            raise ValueError('only ForeignKey fields are admitted here')
+
+        if fk_field.related_model is not subqueryset.model:
+            raise ValueError('related model does not match subquery\'s model')
+
+        self.model = model
+        self.base_qs = queryset
+        self.fk_field = fk_field
+        self.iterate_sortkey = None
+        self.subqueryset = subqueryset
+
+    def _clone(self):
+        """
+        Make a copy of us ready for other operations on our querysets
+        """
+        obj = self.__class__(
+            self.base_qs._chain(),
+            self.fk_field,
+            self.subqueryset._chain(),
+        )
+        obj.iterate_sortkey = self.iterate_sortkey
+        return obj
+
+    def _apply_queryset_method(self, meth, *args, **kwargs):
+        """
+        Apply a queryset method to all members of the chain.
+
+        meth: str, name of queryset method
+
+        The args and kwargs are passed to the method.  Only methods that also
+        return a QuerySet should be used, else the result is unlikely to do any
+        good.  Returns a new instance.
+        """
+        obj = self._clone()
+        qs_meth = getattr(obj.base_qs, meth)
+        obj.base_qs = qs_meth(*args, **kwargs)
+        return obj
+
+    def get_split_querysets(self):
+        """
+        Generate the queryset iterables.
+        """
+        for pk in self.subqueryset.values_list('pk', flat=True):
+            yield self.base_qs.filter(**{self.fk_field.attname: pk})
+
+    # Below we implement a few of the usual queryset methods
+
+    def iterate(self, **kwargs):
+        """
+        Chains QuerySet.interate() calls along split querysets
+
+        kwargs are passed to each Queryset.iterate().
+        """
+        kwargs.setdefault('sortkey', self.iterate_sortkey)
+        it = ((qs.iterate(**kwargs) for qs in self.get_split_querysets()))
+        return chain.from_iterable(it)
+
+    def values_list(self, *args, **kwargs):
+        return self._apply_queryset_method('values_list', *args, **kwargs)
+
+    def prefetch_related(self, *args, **kwargs):
+        return self._apply_queryset_method('prefetch_related', *args, **kwargs)
+
+    def count(self):
+        """
+        Not implemented but declared here to make class pass django_table2's
+        TableQuerySetData.validate().
+        """
+        raise NotImplementedError
+
+    def order_by(self, *field_names):
+        """
+        Not implemented but declared here to make class pass django_table2's
+        TableQuerySetData.validate().
+        """
+        raise NotImplementedError