Can I cast the result of plainto_tsquery to a string using npgsql / ef core to allow searching by part of a word or phrase? - entity-framework-core

Question

My end goal is to be able to do a prefix search in npgsql using ef core, very similar to Npgsql Entity Framework Core PostgreSQL full text search by part of word. And Match a phrase starting with a prefix with full text search explains what the query will look like, something like:
select * from mytable
where
mycolumn ## to_tsquery(CONCAT(CAST(plainto_tsquery('the query') as text), ':*'))
In dotnet, I can get close to this, but not exactly what I want.
Using "Matches" converts to plainto_tsquery:
var results= MyTable
.Where(c => c.MyColumn.Matches("the query"))
.ToList();
which results in:
select * from mytable
where
mycolumn ## plainto_tsquery('query')
Using EF.Functions.ToTsQuery converts to to_tsquery (closer):
var results= MyTable
.Where(c => EF.Functions.ToTsQuery("the query")))
.ToList();
which results in:
select * from mytable
where
mycolumn ## to_tsquery('the query')
And now I want to use to_tsquery, passing in the results of plainto_tsquery and appending the ":*" to make it a prefix, but I can't figure out how to convert plainto_tsquery results to a string. I am trying the following:
var results= MyTable
.Where(c => c.SearchVector.Matches(
EF.Functions.ToTsQuery(
EF.Functions.PlainToTsQuery("the query") + ":*")))
.ToList();
But this fails with:
System.InvalidCastException: Unable to cast object of type 'System.String' to type 'NpgsqlTypes.NpgsqlTsQuery'.
I think I just need to be able to cast the results of EF.Functions.PlainToTsQuery("the query") to a string, but I am not sure how to do that or if possible.
Appreciate any help.
Thanks,
Eric

Answer 1

I was not able to figure out how to cast the results of EF.Functions.PlainToTsQuery("the query") to text, but I was able to solve my issue by creating a stored function which creates the tsquery. And then calling the stored function via ef core.
The stored function is something like the following:
CREATE OR REPLACE FUNCTION public.get_ts_query_for_prefix_search(
query_text text)
RETURNS tsquery
LANGUAGE 'sql'
AS $BODY$
SELECT
CASE plainto_tsquery('simple', query_text) = ''
WHEN true THEN ''
ELSE to_tsquery('simple', CAST(plainto_tsquery('simple', query_text) AS TEXT) || ':*')
END
$BODY$;
And the ef core where clause will be something like the following:
var matches = MyTable
.Where(w => w.MyColumn.Matches(GetTsQueryForPartialSearch(queryText));