Documentation

Convert an entity's key into another entity's.

This function is to be used when you change an entity's Id to be that of another entity. For example:

Bar
  barNum Int
Foo
  bar BarId
  fooNum Int
  Primary bar

In this example, Bar is said to be the BaseEnt(ity), and Foo the child. To model this in Esqueleto, declare:

instance ToBaseId Foo where
  type BaseEnt Foo = Bar
  toBaseIdWitness barId = FooKey barId

Now you're able to write queries such as:

select $
from $ (bar `InnerJoin` foo) -> do
on (toBaseId (foo ^. FooId) ==. bar ^. BarId)
return (bar, foo)

Note: this function may be unsafe to use in conditions not like the one of the example above.

Since: esqueleto-2.4.3

Class that enables one to use toBaseId to convert an entity's key on a query into another (cf. toBaseId).