[backend][models][environment] Move use count to Environment manager

This allows to re-use the annotation in both algorithm and library
models.

beat/web/backend/models/environment.py

@@ -27,6 +27,7 @@
 
 from django.db import models
 from django.core.urlresolvers import reverse
+from django.db.models import Count, Q
 
 from ...code.models import Code
 from ...common.models import Shareable
@@ -45,6 +46,41 @@ class EnvironmentManager(ShareableManager):
         name, version = fullname.rsplit(" ", 1)
         return self.get_by_natural_key(name, version[1:-1])
 
+    def use_count(self, block_list):
+        """Returns a list of environments used for the blocks in that list with
+        how many times each has been used.
+
+        Parameters:
+            block_list (Block) : list of blocks to analyze.
+
+        Returns:
+            list: annotated environments with the number of time they have been
+                used for blocks that are done.
+        """
+
+        from ...experiments.models import Experiment
+        from ...experiments.models import Block
+
+        # Tries to figure through a maximum if blocks in the list have been
+        # successfully used inside an environment.
+
+        # Case 1) The block is part of an experiment that was successful
+        # Case 2) The block is part of an experiment that is not successful
+        # (failed or other), but it is CACHED (if not cached, then we can't
+        # attest anything about the block/environment relationship!)
+
+        return (
+            self.filter(
+                blocks__in=block_list.filter(
+                    Q(experiment__status=Experiment.DONE)
+                    | ((~Q(experiment__status=Experiment.DONE)) & Q(status=Block.DONE))
+                )
+            )
+            .annotate(use_count=Count("id"))
+            .order_by("-creation_date")
+            .distinct()
+        )
+
 
 # ----------------------------------------------------------