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?

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

score 0 · Answer 1 · answered Nov 12 '22 at 19:39

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));

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?

1 Answers1