Working read/write-only fields by truly unhooking them about haskell-opaleye HOT 8 OPEN

Great, in that case maybe we can do them all as versions of a single "write type" that is something like

WriteField a = Omitted | SetToDefault | SetToValue a

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Just to check I understand, do the writing parts of these behave as indicated in the following?

-- Generates "field = value"
requiredTableField :: String -> TableFields (Field_ n a) (Field_ n a)
writeOnlyRequiredTableField :: String -> TableFields (Field_ n a) ()

-- Just: Generates "field = value"
-- Nothing: Generates "field = DEFAULT"
optionalTableField :: String -> TableFields (Maybe (Field_ n a)) (Field_ n a)
writeOnlyOptionalTableField :: String -> TableFields (Maybe (Field_ n a)) ()

-- Generates nothing for "field"
readOnlyTableField :: String -> TableFields () (Field_ n a)
missingTableField :: TableFields () ()

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Correct!

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I just noticed a related issue on rel8: circuithub/rel8#193

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Yeah I think many Haskell db libraries handle this type of stuff poorly. I don't think persistent handles default/generated columns well either.

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As a side note readOnlyTableField does actually work correctly for generated columns due to them allowing the keyword DEFAULT for inserts and updates even though all other values are rejected due to their read-only nature.

For updates, Opaleye generates a syntactically valid query that will actually "typecheck" against the schema, so to speak, but unless the DEFAULT value is a constant, the semantics of that query is likely not what you want:

db=# create temp table t(
  id integer primary key generated always as identity,
  created_at timestamptz not null default now(),
  v integer not null
);
CREATE TABLE

db=# insert into t(v) values(1);
INSERT 0 1

db=# select * from t;
 id |          created_at           | v
----+-------------------------------+---
  1 | 2024-05-20 08:13:51.872027+00 | 1

db=# update t set v = 2;
UPDATE 1

db=# select * from t;
 id |          created_at           | v
----+-------------------------------+---
  1 | 2024-05-20 08:13:51.872027+00 | 2

db=# update t set v = 3, id = default, created_at = default;
UPDATE 1

db=# select * from t;
 id |          created_at           | v
----+-------------------------------+---
  2 | 2024-05-20 08:14:21.103436+00 | 3

The query updates the field by recomputing the DEFAULT expression. In particular, this means that a table with a DEFAULTed foreign key target cannot be updated in this manner (since that would violate referential integrity).

(Which is just to say that the "nothing generated, not even DEFAULT" case is vital.)

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Ah, I see this was mentioned in #590 (comment) and you were talking about generated columns, not the…serial thing.

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Yes just GENERATED ALWAYS AS ( generation_expr ) STORED

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