Add migration to convert GA timezone to UTC

We are ignoring the timezone from GA requests so previous data needs to
be converted to account for this.

See https://www.pivotaltracker.com/story/show/62779118 for details.

migrations/008_convert_ga_buckets_to_utc.py

@@ -0,0 +1,90 @@
+"""
+Convert Google Analytics buckets from Europe/London to UTC
+"""
+import logging
+import copy
+from itertools import imap, ifilter
+from datetime import timedelta
+
+from backdrop.core.records import Record
+from backdrop.core.timeutils import utc
+
+
+log = logging.getLogger(__name__)
+
+GA_BUCKETS_TO_MIGRATE = [
+    "carers_allowance_journey",
+    "deposit_foreign_marriage_journey",
+    "pay_foreign_marriage_certificates_journey",
+    "pay_legalisation_drop_off_journey",
+    "pay_legalisation_post_journey",
+    "pay_register_birth_abroad_journey",
+    "pay_register_death_abroad_journey",
+    "lpa_journey",
+    "licensing_journey",
+]
+
+
+def fix_timestamp(document):
+    """Return a new dict with the _timestamp field fixed
+    """
+    document = copy.deepcopy(document)
+    document['_timestamp'] = document['_timestamp'].replace(tzinfo=None) + timedelta(hours=1)
+    return document
+
+
+def strip_internal_fields(document):
+    """Return a new dict with all internal fields removed
+
+    Leaves _timestamp and _id in place
+    """
+    def allowed_field(key):
+        return not key.startswith('_') or key in ['_timestamp', '_id']
+
+    return dict(
+        (key, value) for key, value in document.items() if allowed_field(key))
+
+
+def is_bst(document):
+    """Return true if a document looks like it's BST"""
+    return document['_timestamp'].hour == 23
+
+
+def create_record(document):
+    """Return a dict with internal fields applied"""
+    return Record(document).to_mongo()
+
+
+def fix_id(document):
+    """Return a new dict with the _id field recalculated"""
+    def _format(timestamp):
+        return to_utc(timestamp).strftime("%Y%m%d%H%M%S")
+
+    def data_id(data_type, timestamp, period):
+        """_id generation function copied from backdrop-ga-collector"""
+        return base64.urlsafe_b64encode("_".join(
+            [data_type, _format(timestamp), period]))
+
+    document = copy.deepcopy(document)
+    document['_id'] = data_id(
+        document['dataType'], document['_timestamp'],
+        document['timeSpan'])
+
+    return document
+
+def up(db):
+    for name in GA_BUCKETS_TO_MIGRATE:
+        collection = db.get_repository(name)
+
+        documents = collection.find({})
+
+        documents = ifilter(is_bst, documents)
+        documents = imap(strip_internal_fields, documents)
+        documents = imap(fix_timestamp, documents)
+        documents = map(create_record, documents)
+
+        if len(documents) > 0:
+            log.info("Convert GA timezone: {0}".format(name))
+
+        map(collection.save, documents)
+