feat: expose unique columns and many-to-many markers in OpenAPI

Add unique constraint and many-to-many relationship metadata to the
generated OpenAPI spec so clients can render them.

- Store unique constraints on Table as tableUniqueCols (mirroring
  tablePKCols) instead of denormalizing them onto each Column.
- Compute unique constraints via a per-table tbl_unique_cols CTE in
  tablesSqlQuery.
- Annotate unique columns and composite unique constraints in property
  descriptions, and emit m2m markers in table descriptions.

src/library/PostgREST/Response/OpenAPI.hs

@@ -30,8 +30,8 @@ import PostgREST.Network                  (escapeHostName)
 import PostgREST.Query.OpenApi            (TableAccess (..), TablesAccess)
 import PostgREST.SchemaCache              (SchemaCache (..))
 import PostgREST.SchemaCache.Identifiers  (FieldName, QualifiedIdentifier (..))
-import PostgREST.SchemaCache.Relationship (Cardinality (..), Relationship (..),
-                                           RelationshipsMap)
+import PostgREST.SchemaCache.Relationship (Cardinality (..), Junction (..),
+                                           Relationship (..), RelationshipsMap)
 import PostgREST.SchemaCache.Routine      (FuncVolatility (..), Routine (..),
                                            RoutineParam (..))
 import PostgREST.SchemaCache.Table        (Column (..), Table (..), TablesMap,
@@ -110,13 +110,35 @@ parseDefault colType colDefault =
 makeTableDef :: RelationshipsMap -> (Table, TableAccess) -> (Text, Schema)
 makeTableDef rels (t, access) =
   (tn, (mempty :: Schema)
-    & description .~ tableDescription t
+    & description .~ tblDescription
     & type_ ?~ SwaggerObject
     & properties .~ fromList (makeProperty t rels <$> cols)
     & required .~ fmap colName (filter (not . colNullable) cols))
   where
     tn = tableName t
     cols = accessibleCols t (taSelectCols access)
+    tblDescription = case m2mMarkers t rels of
+      []  -> tableDescription t
+      ms  -> Just $ maybe "" (`T.append` "\n\n") (tableDescription t) <> T.intercalate "\n" ms
+
+-- | Emits markers for the many-to-many relationships of a table, so that clients
+-- can render these relations. The marker includes the target table(embedding key),
+-- the junction table and the junction columns referencing source and target.
+m2mMarkers :: Table -> RelationshipsMap -> [Text]
+m2mMarkers tbl rels = mapMaybe m2mMarker searchedRels
+  where
+    searchedRels = fromMaybe mempty $ HM.lookup (QualifiedIdentifier (tableSchema tbl) (tableName tbl), tableSchema tbl) rels
+    m2mMarker Relationship{relForeignTable, relCardinality=M2M junction} =
+      Just $ T.intercalate ""
+        [ "<m2m table='", qiName relForeignTable
+        , "' junction='", qiName (junTable junction)
+        , "' source='", junctionSourceCol junction
+        , "' target='", junctionTargetCol junction
+        , "'/>"
+        ]
+    m2mMarker _ = Nothing
+    junctionSourceCol junction = maybe mempty snd (headMay $ junColsSource junction)
+    junctionTargetCol junction = maybe mempty snd (headMay $ junColsTarget junction)
 
 accessibleCols :: Table -> [FieldName] -> [Column]
 accessibleCols t cols = filter ((`elem` cols) . colName) (tableColumnsList t)
@@ -144,11 +166,18 @@ makeProperty tbl rels col = (colName col, Inline s)
         (\(a, b) -> T.intercalate "" ["This is a Foreign Key to `", a, ".", b, "`.<fk table='", a, "' column='", b, "'/>"]) <$> fTblCol
     pk :: Bool
     pk = colName col `elem` tablePKCols tbl
+    uniqueNotes :: [Text]
+    uniqueNotes = mapMaybe uniqueNote (filter (colName col `elem`) (tableUniqueCols tbl))
+      where
+        uniqueNote cols
+          | length cols == 1 = Just "This is a Unique column.<unique/>"
+          | otherwise        = Just $ "This is part of a composite unique constraint.<unique cols='" <> T.intercalate "," cols <> "'/>"
     n = catMaybes
       [ Just "Note:"
       , if pk then Just "This is a Primary Key.<pk/>" else Nothing
-      , fk
       ]
+      <> uniqueNotes
+      <> catMaybes [fk]
     d =
       if length n > 1 then
         Just $ T.append (maybe "" (`T.append` "\n\n") $ colDescription col) (T.intercalate "\n" n)

src/library/PostgREST/SchemaCache.hs

@@ -232,6 +232,7 @@ decodeTables =
     <*> column HD.bool
     <*> column HD.bool
     <*> arrayColumn HD.text
+    <*> column (HD.refine parseUniqueCols HD.jsonb)
     <*> parseCols (compositeArrayColumn
       (Column
         <$> compositeField HD.text
@@ -247,6 +248,12 @@ decodeTables =
 parseCols :: HD.Row [Column] -> HD.Row ColumnMap
 parseCols = fmap (HMI.fromList . map (\col@Column{colName} -> (colName, col)))
 
+parseUniqueCols :: JSON.Value -> Either Text [[FieldName]]
+parseUniqueCols val =
+  case JSON.fromJSON val of
+    JSON.Success cols -> Right cols
+    JSON.Error err    -> Left ("Invalid unique columns: " <> T.pack err)
+
 decodeRels :: HD.Result [Relationship]
 decodeRels =
  HD.rowList relRow
@@ -676,6 +683,25 @@ tablesSqlQuery pgVer =
       AND NOT pg_is_other_temp_schema(r.relnamespace)
       AND NOT a.attisdropped
     GROUP BY r.oid
+  ),
+  tbl_unique_cols AS (
+    SELECT
+      r.oid AS relid,
+      jsonb_agg(cols ORDER BY c.oid) AS unique_cols
+    FROM pg_class r
+    JOIN pg_constraint c
+      ON r.oid = c.conrelid
+    JOIN LATERAL (
+      SELECT jsonb_agg(a.attname::text ORDER BY k.ord) AS cols
+      FROM unnest(c.conkey) WITH ORDINALITY AS k(attnum, ord)
+      JOIN pg_attribute a ON a.attrelid = r.oid AND a.attnum = k.attnum
+    ) col_info ON TRUE
+    WHERE
+      c.contype = 'u'
+      AND r.relkind IN ('r', 'p')
+      AND r.relnamespace NOT IN ('pg_catalog'::regnamespace, 'information_schema'::regnamespace)
+      AND NOT pg_is_other_temp_schema(r.relnamespace)
+    GROUP BY r.oid
   )
   SELECT
     n.nspname AS table_schema,
@@ -709,11 +735,13 @@ tablesSqlQuery pgVer =
       )
     ) AS deletable,
     coalesce(tpks.pk_cols, '{}') as pk_cols,
+    coalesce(tunq.unique_cols, '[]'::jsonb) as unique_cols,
     coalesce(cols_agg.columns, '{}') as columns
   FROM pg_class c
   JOIN pg_namespace n ON n.oid = c.relnamespace
   LEFT JOIN pg_description d on d.objoid = c.oid and d.objsubid = 0 and d.classoid = 'pg_class'::regclass
   LEFT JOIN tbl_pk_cols tpks ON c.oid = tpks.relid
+  LEFT JOIN tbl_unique_cols tunq ON c.oid = tunq.relid
   LEFT JOIN columns_agg cols_agg ON c.oid = cols_agg.relid
   WHERE c.relkind IN ('v','r','m','f','p')
   AND c.relnamespace NOT IN ('pg_catalog'::regnamespace, 'information_schema'::regnamespace)

src/library/PostgREST/SchemaCache/Table.hs

@@ -29,6 +29,10 @@ data Table = Table
   , tableUpdatable   :: Bool
   , tableDeletable   :: Bool
   , tablePKCols      :: [FieldName]
+    -- ^ Each element is the position-ordered column list of a unique
+    -- constraint. A single-column unique constraint is represented by a
+    -- single-element list.
+  , tableUniqueCols  :: [[FieldName]]
   , tableColumns     :: ColumnMap
   }
   deriving (Show, Generic, JSON.ToJSON)