Using unnest as a field rather than a table in jOOQ - postgresql

Question

This is the query I am trying to run in PostgreSQL:
SELECT * FROM message WHERE id IN (
SELECT unnest(message_ids) "mid"
FROM session_messages WHERE session_id = '?' ORDER BY "mid" ASC
);
However, I am not able do something:
create.selectFrom(Tables.MESSAGE).where(Tables.MESSAGE.ID.in(
create.select(DSL.unnest(..))
Because DSL.unnest is a Table<?>, which makes sense since it is trying to take a List-like object (mostly a literal) and convert it to table.
I have a feeling that I need to find a way to wrap the function around my field name, but I have no clue as to how to proceed.
NOTE. The field message_ids is of type bigint[].
EDIT
So, this is how I am doing it now, and it works exactly as expected, but I am not sure if this is the best way to do it:
Field<Long> unnestMessageIdField = DSL.field(
"unnest(" + SESSION_MESSAGES.MESSAGE_IDS.getName() + ")",
Long.class)
.as("mid");
Field<Long> messageIdField = DSL.field("mid", Long.class);
MESSAGE.ID.in(
ctx.select(messageIdField).from(
ctx.select(unnestMessageIdField)
.from(Tables.CHAT_SESSION_MESSAGES)
.where(Tables.CHAT_SESSION_MESSAGES.SESSION_ID.eq(sessionId))
)
.where(condition)
)
EDIT2
After going through the code on https://github.com/jOOQ/jOOQ/blob/master/jOOQ/src/main/java/org/jooq/impl/DSL.java I guess the right way to do this would be:
DSL.function("unnest", SQLDataTypes.BIGINT.getArrayType(), SESSION_MESSAGES.MESSAGE_IDS)
EDIT3
Since as always lukas is here for my jOOQ woes, I am going to capitalize on this :)
Trying to generalize this function, in a signature of sort
public <T> Field<T> unnest(Field<T[]> arrayField) {
return DSL.function("unnest", <??>, arrayField);
}
I don't know how I can fetch the datatype. There seems to be a way to get DataType<T[]> from DataType<T> using DataType::getArrayDataType(), but the reverse is not possible. There is this class I found ArrayDataType, but it seems to be package-private, so I cannot use it (and even if I could, it does not expose the field elementType).

Answer 1

Old PostgreSQL versions had this funky idea that it is OK to produce a table from within the SELECT clause, and expand it into the "outer" table, as if it were declared in the FROM clause. That is a very obscure PostgreSQL legacy, and this example is a good chance to get rid of it, and use LATERAL instead. Your query is equivalent to this one:
SELECT *
FROM message
WHERE id IN (
SELECT "mid"
FROM session_messages
CROSS JOIN LATERAL unnest(message_ids) AS t("mid")
WHERE session_id = '?'
);
This can be translated to jOOQ much more easily as:
DSL.using(configuration)
.select()
.from(MESSAGE)
.where(MESSAGE.ID).in(
select(field(name("mid"), MESSAGE.ID.getDataType()))
.from(SESSION_MESSAGES)
.crossJoin(lateral(unnest(SESSION_MESSAGES.MESSAGE_IDS)).as("t", "mid"))
.where(SESSION_MESSAGES.SESSION_ID.eq("'?'"))
)

Answer 2

The Edit3 in the question is quite close to a decent solution for this problem.
We can create a custom generic unnest method for jOOQ which accepts Field and use it in jOOQ query normally.
Helper method:
public static <T> Field<T> unnest(Field<T[]> field) {
var type = (Class<T>) field.getType().getComponentType();
return DSL.function("unnest", type, field);
}
Usage:
public void query(SessionId sessionId) {
var field = unnest(SESSION_MESSAGES.MESSAGE_IDS, UUID.class);
dsl.select().from(MESSAGE).where(
MESSAGE.ID.in(
dsl.select(field).from(SESSION_MESSAGES)
.where(SESSION_MESSAGES.SESSION_ID.eq(sessionId.id))
.orderBy(field)
)
);
}