piq9117 - Defining composite keys in beam

When I’m trying to define a table in beam to represent my database table, I always forget how to define it. So I’m writing it down here. Typically when defining a table in beam it will look like this

data UserT = User
  { id :: Columnar f Int32,
    username :: Columnar f Text,
    firstName :: Columnar f Text,
    lastName :: Columnar f Text
  }

Then defining it’s primary key will look like this

instance Table UserT where
  newtype PrimaryKey UserT f = UserId (Columnar f Int32)
    deriving (Generic)
    deriving (Beamable)
  primaryKey = UserId <<< id

However, for composite keys it needs to be a little different. Remember, newtype can only represent one value. To represent more than one value, use data definition. Let’s say the primary key for this table will be username, first name, and last name.

data UserT = User
  { username :: Columnar f Text,
    firstName :: Columnar f Text,
    lastName :: Columnar f Text
  }

instance Table UserT where
  data PrimaryKey UserT f 
    = UserId 
        (Columnar f Text) 
        (Columnar f Text) 
        (Columnar f Text)
    deriving (Generic)
    deriving (Beamable)
  primaryKey table = 
    UserId table.username table.firstName table.lastName

There you have it, future me! I hope you won’t struggle trying to forage for this information.