Derived/Computed Columns

[parsonsmatt]

Postgres 12 supports generated columns. We should be able to support these.

The current ability to support is pretty limited. Per the Postgres docs,

A generated column cannot be written to directly. In INSERT or UPDATE commands, a value cannot be specified for a generated column, but the keyword DEFAULT may be specified.

So if we have a model definition like:

User
    name Text
    birthday Day
    age Int GeneratedAsDifferenceFromAgeToBirthday Virtual

Then we would currently need to provide some dummy value to this field when performing writes/updates, like:

insert User 
    { userName = "Bob"
    , userBirthday = fromGregorian 1988 29 09
    , userAge = 123456789
    }

The inserting code would see that the column is generated and use DEFAULT instead of a value.

The design in #1037 makes this trivial - generated columns would be present on the full entity, while New entity would not have them, and (importantly) Partial entity would not either.

[parsonsmatt]

Support for this was added in 2.11, thanks to #1122