[FIX] prefetch issues on computed fields (#18799)

When deciding to prefetch records (getting records from the cache with no value
for the field being fetched), if the field was computed `determine_value` would
just get all records, not limited by the normal prefetch limit; for large
recordsets this would generate gigantic prefetch lists for records we may not
need at all.

Fix by applying the `PREFETCH_MAX` limit to records from the cache as is done
in `_prefetch_field`.

openerp/models.py

@@ -3206,9 +3206,6 @@ class BaseModel(object):
         # fetch the records of this model without field_name in their cache
         records = self._in_cache_without(field)
 
-        if len(records) > PREFETCH_MAX:
-            records = records[:PREFETCH_MAX] | self
-
         # determine which fields can be prefetched
         if not self.env.in_draft and \
                 self._context.get('prefetch_fields', True) and \
@@ -5675,16 +5672,19 @@ class BaseModel(object):
         return RecordCache(self)
 
     @api.model
-    def _in_cache_without(self, field):
-        """ Make sure ``self`` is present in cache (for prefetching), and return
-            the records of model ``self`` in cache that have no value for ``field``
-            (:class:`Field` instance).
+    def _in_cache_without(self, field, limit=PREFETCH_MAX):
+        """ Return records to prefetch that have no value in cache for ``field``
+            (:class:`Field` instance), including ``self``.
+            Return at most ``limit`` records.
         """
         env = self.env
         prefetch_ids = env.prefetch[self._name]
         prefetch_ids.update(self._ids)
         ids = filter(None, prefetch_ids - set(env.cache[field]))
-        return self.browse(ids)
+        recs = self.browse(ids)
+        if limit and len(recs) > limit:
+            recs = self + (recs - self)[:(limit - len(self))]
+        return recs
 
     @api.model
     def refresh(self):