Singular response for PATCH request (#634)

* Allow a singular entity to be returned from an update

Since a `PATCH` will be an update that could affect many rows, there might be
more than one object returned. By allowing asking for a singular response,
the/an object will be returned instead of a list. This is useful in cases when
the `PATCH` is against single entity (i.e. `/items?pkey=eq.99`).

* Add tests for plurality=singular for `PATCH` requests

* Disallow updating more than one row if `plurality=singular`

As discussed in #634, we don't want to allow updating several rows with
a `PATCH` request when the `Prefer` header specifies
`return=representation;plurality=singular` as this would almost
certainly be a client error.

* Add test for patching multiple objects with singular response

Patching > 1 object with `return=representation;plurality=singular`
should return `400 Bad Request`.

* Only add singleton range for read API requests

* Disallow inserting more than one row if `plurality=singular`

Disallow inserting several rows with a `POST` request when the `Prefer`
header specifies `return=representation;plurality=singular` as this
would almost certainly be a client error.

* Only import `q` from `Text.InterpolatedString.Perl6`

* Update OpenAPI response for PATCH/POST to mention `plurality=singular`

The OpenAPI response for PATCH/POST requests now includes the `Prefer` value
`return=representation;plurality=singular`.

* Add entry in changelog

src/PostgREST/ApiRequest.hs

@@ -190,7 +190,7 @@ userApiRequest schema req reqBody =
   endingIn xx key = lastWord `elem` xx
     where lastWord = last $ T.split (=='.') key
 
-  headerRange = if singular then singletonRange 0 else rangeRequested hdrs
+  headerRange = if singular && method == "GET" then singletonRange 0 else rangeRequested hdrs
   urlOffsetRange = rangeGeq . fromMaybe (0::Integer) $
     readMaybe =<< join (lookup "offset" qParams)
   urlRange = restrictRange

src/PostgREST/App.hs

@@ -21,6 +21,8 @@ import qualified Hasql.Transaction         as HT
 import           Text.Parsec.Error
 import           Text.ParserCombinators.Parsec (parse)
 
+import qualified Text.InterpolatedString.Perl6 as P6 (q)
+
 import           Network.HTTP.Types.Header
 import           Network.HTTP.Types.Status
 import           Network.HTTP.Types.URI (renderSimpleQuery)
@@ -129,6 +131,13 @@ app dbStructure conf apiRequest =
         Left e -> return $ responseLBS status400 [jsonH] $ toS e
         Right (sq,mq) -> do
           let isSingle = (==1) $ V.length rows
+          when (not isSingle && iPreferSingular apiRequest) $
+            HT.sql [P6.q| DO $$
+                       BEGIN RAISE EXCEPTION cardinality_violation
+                       USING MESSAGE =
+                         'plurality=singular specified, but more than one object would be inserted';
+                       END $$;
+                     |]
           let pKeys = map pkName $ filter (filterPk schema table) allPrKeys -- would it be ok to move primary key detection in the query itself?
           let stm = createWriteStatement qi sq mq isSingle (iPreferRepresentation apiRequest) pKeys (contentType == TextCSV) payload
           row <- H.query uniform stm
@@ -145,10 +154,19 @@ app dbStructure conf apiRequest =
       case mutateSqlParts of
         Left e -> return $ responseLBS status400 [jsonH] $ toS e
         Right (sq,mq) -> do
-          let stm = createWriteStatement qi sq mq False (iPreferRepresentation apiRequest) [] (contentType == TextCSV) payload
+          let singular = iPreferSingular apiRequest
+          let representation = iPreferRepresentation apiRequest
+          let stm = createWriteStatement qi sq mq singular representation [] (contentType == TextCSV) payload
           row <- H.query uniform stm
           let (_, queryTotal, _, body) = extractQueryResult row
-              r = contentRangeH 0 (toInteger $ queryTotal-1) (toInteger <$> Just queryTotal)
+          when (singular && queryTotal > 1) $
+            HT.sql [P6.q| DO $$
+                       BEGIN RAISE EXCEPTION cardinality_violation
+                       USING MESSAGE =
+                         'plurality=singular specified, but more than one object would be updated';
+                       END $$;
+                     |]
+          let r = contentRangeH 0 (toInteger $ queryTotal-1) (toInteger <$> Just queryTotal)
               s = case () of _ | queryTotal == 0 -> status404
                                | iPreferRepresentation apiRequest == Full -> status200
                                | otherwise -> status204

src/PostgREST/OpenAPI.hs

@@ -162,13 +162,10 @@ makeGetParams cs =
   , makePreferParam ["plurality=singular", "count=none"]
   ]
 
-makeReturnPreferenceParam :: Param
-makeReturnPreferenceParam =
-  makePreferParam ["return=representation", "return=minimal", "return=none"]
-
 makePostParams :: Text -> [Param]
 makePostParams tn =
-  [ makeReturnPreferenceParam
+  [ makePreferParam ["return=representation", "return=representation;plurality=singular",
+                     "return=minimal", "return=none"]
   , (mempty :: Param)
     & name        .~ "body"
     & description ?~ tn
@@ -178,7 +175,7 @@ makePostParams tn =
 
 makeDeleteParams :: [Param]
 makeDeleteParams =
-  [ makeReturnPreferenceParam ]
+  [ makePreferParam ["return=representation", "return=minimal", "return=none"] ]
 
 makePathItem :: (Table, [Column], [Text]) -> (FilePath, PathItem)
 makePathItem (t, cs, _) = ("/" ++ unpack tn, p $ tableInsertable t)