diff --git a/Database/Persist/Class/PersistEntity.hs b/Database/Persist/Class/PersistEntity.hs
--- a/Database/Persist/Class/PersistEntity.hs
+++ b/Database/Persist/Class/PersistEntity.hs
@@ -15,8 +15,10 @@
 import Database.Persist.Types.Base
 import Database.Persist.Class.PersistField
 import Data.Text (Text)
+import qualified Data.Text as T
 import Data.Aeson (ToJSON (..), FromJSON (..), object, (.:), (.=), Value (Object))
 import Control.Applicative ((<$>), (<*>))
+import Data.Monoid (mappend)
 
 -- | A single database entity. For example, if writing a blog application, a
 -- blog entry would be an entry, containing fields such as title and content.
@@ -74,10 +76,10 @@
 -- Since 1.1.0
 type Key val = KeyBackend (PersistEntityBackend val) val
 
--- | Datatype that represents an entity, with both its key and
+-- | Datatype that represents an entity, with both its 'Key' and
 -- its Haskell representation.
 --
--- When using the an SQL-based backend (such as SQLite or
+-- When using a SQL-based backend (such as SQLite or
 -- PostgreSQL), an 'Entity' may take any number of columns
 -- depending on how many fields it has. In order to reconstruct
 -- your entity on the Haskell side, @persistent@ needs all of
@@ -119,3 +121,29 @@
         <$> o .: "key"
         <*> o .: "value"
     parseJSON _ = fail "FromJSON Entity: not an object"
+
+instance PersistField entity => PersistField (Entity entity) where
+    toPersistValue (Entity k v) = case toPersistValue v of
+        (PersistMap alist) -> PersistMap ((idField, toPersistValue k) : alist)
+        _ -> error $ T.unpack $ errMsg "expected PersistMap"
+
+    fromPersistValue (PersistMap alist) = case after of
+        [] -> Left $ errMsg $ "did not find " `mappend` idField `mappend` " field"
+        ("_id", k):afterRest ->
+            case fromPersistValue (PersistMap (before ++ afterRest)) of
+                Right record -> Right $ Entity (Key k) record
+                Left err     -> Left err
+        _ -> Left $ errMsg $ "impossible id field: " `mappend` T.pack (show alist)
+      where
+        (before, after) = break ((== idField) . fst) alist
+
+    fromPersistValue x = Left $
+          errMsg "Expected PersistMap, received: " `mappend` T.pack (show x)
+
+errMsg :: Text -> Text
+errMsg = mappend "PersistField entity fromPersistValue: "
+
+-- | Realistically this is only going to be used for MongoDB,
+-- so lets use MongoDB conventions
+idField :: Text
+idField = "_id"
diff --git a/Database/Persist/Class/PersistUnique.hs b/Database/Persist/Class/PersistUnique.hs
--- a/Database/Persist/Class/PersistUnique.hs
+++ b/Database/Persist/Class/PersistUnique.hs
@@ -5,6 +5,7 @@
     , getByValue
     , insertBy
     , replaceUnique
+    , checkUnique
     ) where
 
 import qualified Prelude
@@ -107,7 +108,7 @@
           Nothing -> replace key datumNew >> return Nothing
           (Just conflictingKey) -> return $ Just conflictingKey
       where
-        changedKeys = uniqueKeysOriginal \\ uniqueKeysNew
+        changedKeys = uniqueKeysNew \\ uniqueKeysOriginal
         uniqueKeysOriginal = persistUniqueKeys original
 
 -- | Check whether there are any conflicts for unique keys with this entity and
diff --git a/Database/Persist/Sql/Class.hs b/Database/Persist/Sql/Class.hs
--- a/Database/Persist/Sql/Class.hs
+++ b/Database/Persist/Sql/Class.hs
@@ -312,3 +312,8 @@
         _mn = return n `asTypeOf` a
 instance PersistFieldSql Rational where
     sqlType _ = SqlNumeric 22 12   --  FIXME: Ambigous, 12 is from Pico which is used to convert Rational to number string
+
+-- perhaps a SQL user can figure this sqlType out?
+-- It is really intended for MongoDB though.
+instance PersistField entity => PersistFieldSql (Entity entity) where
+    sqlType _ = SqlOther "embedded entity, hard to type"
diff --git a/persistent.cabal b/persistent.cabal
--- a/persistent.cabal
+++ b/persistent.cabal
@@ -1,5 +1,5 @@
 name:            persistent
-version:         1.2.2.0
+version:         1.2.3.0
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>