Docs on Fixing Postgres Default Value Mismatches Is Incorrect

[prikhi]

Sometimes you'll write a model file that has a default attr like default=FALSE and get constant no-op migration suggestions because persistent is expecting your model definition to be default=false.

In the Quasi module docs(https://hackage-content.haskell.org/package/persistent-2.16.0.0/docs/Database-Persist-Quasi.html), there is a little blurb about using \d+ in psql to get the proper value to put in default= to make these go away.

But this is not always accurate, apparently psql does a little bit of cleanup when rendering the defaults. The real way is to mimic persistent's query(

persistent/persistent-postgresql/Database/Persist/Postgresql.hs

Lines 794 to 809 in 9723907

	getColumns getter def cols = do
	let sqlv = T.concat
	[ "SELECT "
	, "column_name "
	, ",is_nullable "
	, ",COALESCE(domain_name, udt_name)" -- See DOMAINS below
	, ",column_default "
	, ",generation_expression "
	, ",numeric_precision "
	, ",numeric_scale "
	, ",character_maximum_length "
	, "FROM information_schema.columns "
	, "WHERE table_catalog=current_database() "
	, "AND table_schema=current_schema() "
	, "AND table_name=? "
	]

) and pull the value from there:

SELECT 
  column_default
FROM 
  information_schema.columns
WHERE
  table_name='YOUR_TABLE_NAME'
AND
  column_name='YOUR_COLUMN_NAME'

One example is psql showing a default of now() + '120 days'::interval while persistent expects an attr of default="(now() + '120 days'::interval)".

Would be nice to mention this other query in the persistent docs.