add basic ARCHITECTURE.md (#2503)

* refactor: move ApiRequest a top-level module
* refactor: rename DbStructure to SchemaCache
* refactor: GucHeader inside Response
* refactor: admin app to Workers

src/PostgREST/SchemaCache/Relationship.hs

@@ -0,0 +1,64 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric  #-}
+
+module PostgREST.SchemaCache.Relationship
+  ( Cardinality(..)
+  , Relationship(..)
+  , Junction(..)
+  , RelationshipsMap
+  ) where
+
+import qualified Data.Aeson          as JSON
+import qualified Data.HashMap.Strict as HM
+
+import PostgREST.SchemaCache.Identifiers (FieldName,
+                                          QualifiedIdentifier, Schema)
+
+import Protolude
+
+
+-- | Relationship between two tables.
+data Relationship = Relationship
+  { relTable        :: QualifiedIdentifier
+  , relForeignTable :: QualifiedIdentifier
+  , relIsSelf       :: Bool -- ^ Whether is a self relationship
+  , relCardinality  :: Cardinality
+  , relTableIsView  :: Bool
+  , relFTableIsView :: Bool
+  }
+  | ComputedRelationship
+  { relFunction     :: QualifiedIdentifier
+  , relTable        :: QualifiedIdentifier
+  , relForeignTable :: QualifiedIdentifier
+  , relToOne        :: Bool
+  , relIsSelf       :: Bool
+  }
+  deriving (Eq, Ord, Generic, JSON.ToJSON)
+
+-- | The relationship cardinality
+-- | https://en.wikipedia.org/wiki/Cardinality_(data_modeling)
+data Cardinality
+  = O2M {relCons :: FKConstraint, relColumns :: [(FieldName, FieldName)]}
+  -- ^ one-to-many
+  | M2O {relCons :: FKConstraint, relColumns :: [(FieldName, FieldName)]}
+  -- ^ many-to-one
+  | O2O {relCons :: FKConstraint, relColumns :: [(FieldName, FieldName)]}
+  -- ^ one-to-one, this is a refinement over M2O so operating on it is pretty much the same as M2O
+  | M2M Junction
+  -- ^ many-to-many
+  deriving (Eq, Ord, Generic, JSON.ToJSON)
+
+type FKConstraint = Text
+
+-- | Junction table on an M2M relationship
+data Junction = Junction
+  { junTable       :: QualifiedIdentifier
+  , junConstraint1 :: FKConstraint
+  , junConstraint2 :: FKConstraint
+  , junColumns1    :: [(FieldName, FieldName)]
+  , junColumns2    :: [(FieldName, FieldName)]
+  }
+  deriving (Eq, Ord, Generic, JSON.ToJSON)
+
+-- | Key based on the source table and the foreign table schema
+type RelationshipsMap = HM.HashMap (QualifiedIdentifier, Schema)  [Relationship]