Import docs patterns #13

docs/user/patterns.rst

@@ -1,4 +1,46 @@
 Common Patterns
 ===============
 
-TODO
+DynamoDB Local
+--------------
+
+Connect to a local DynamoDB instance.
+
+::
+
+    import boto3
+    import bloop
+
+    boto_client = boto3.client(
+        "dynamodb",
+        endpoint="http://127.0.0.1:8000",
+        region_name="us-west-2")
+    client = bloop.Client(boto_client=boto_client)
+    engine = bloop.Engine(client=client)
+
+.. note::
+
+    DynamoDB Local has an issue with expressions and Global Secondary Indexes, and will throw errors about
+    ExpressionAttributeName when you query or scan against a GSI.  For example, see
+    `this issue <https://github.com/numberoverzero/bloop/issues/43>`_.
+
+Generic "if not exist"
+----------------------
+
+Condition to ensure an object's hash (or hash + range) key are not set (item doesn't exist).
+
+::
+
+    def if_not_exist(obj):
+        hash_key = obj.Meta.hash_key
+        range_key = obj.Meta.range_key
+
+        condition = hash_key.is_(None)
+        if range_key:
+            condition &= range_key.is_(None)
+        return condition
+
+
+    # Usage
+    tweet = Tweet(account=uuid.uuid4(), id="numberoverzero", ...)
+    engine.save(tweet, condition=if_not_exist(tweet))