refactor: Relation to Relationship

Also remove the unused UnknownRelationship error.

src/PostgREST/DbStructure/Relationship.hs

@@ -0,0 +1,64 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric  #-}
+
+module PostgREST.DbStructure.Relationship
+  ( Cardinality(..)
+  , ForeignKey(..)
+  , PrimaryKey(..)
+  , Relationship(..)
+  , Junction(..)
+  , isSelfReference
+  ) where
+
+import qualified Data.Aeson as JSON
+
+import PostgREST.DbStructure.Table (Column (..), ForeignKey (..),
+                                    Table (..))
+
+import Protolude
+
+
+-- | Relationship between two tables.
+--
+-- The order of the relColumns and relForeignColumns should be maintained to get the
+-- join conditions right.
+--
+-- TODO merge relColumns and relForeignColumns to a tuple or Data.Bimap
+data Relationship = Relationship
+  { relTable          :: Table
+  , relColumns        :: [Column]
+  , relForeignTable   :: Table
+  , relForeignColumns :: [Column]
+  , relCardinality    :: Cardinality
+  }
+  deriving (Eq, Generic, JSON.ToJSON)
+
+-- | The relationship cardinality
+-- | https://en.wikipedia.org/wiki/Cardinality_(data_modeling)
+-- TODO: missing one-to-one
+data Cardinality
+  = O2M FKConstraint -- ^ one-to-many cardinality
+  | M2O FKConstraint -- ^ many-to-one cardinality
+  | M2M Junction     -- ^ many-to-many cardinality
+  deriving (Eq, Generic, JSON.ToJSON)
+
+type FKConstraint = Text
+
+-- | Junction table on an M2M relationship
+data Junction = Junction
+  { junTable       :: Table
+  , junConstraint1 :: FKConstraint
+  , junColumns1    :: [Column]
+  , junConstraint2 :: FKConstraint
+  , junColumns2    :: [Column]
+  }
+  deriving (Eq, Generic, JSON.ToJSON)
+
+isSelfReference :: Relationship -> Bool
+isSelfReference r = relTable r == relForeignTable r
+
+data PrimaryKey = PrimaryKey
+  { pkTable :: Table
+  , pkName  :: Text
+  }
+  deriving (Generic, JSON.ToJSON)