Postgres function - using table mapping to update value - postgresql

Question

I have a general question. I have a function that creates a file. However, within that function presently I am hard coding the file name pattern based on argument inputs. Now I have come to a point where I need to have more than one file name pattern. I devised a way of using another table as the file name map that the function can simply call if the user inputs that file name pattern id. Here is my example to help better illustrate my point:
Here is the table creation and data insertion for referential purposes:
create table some_schema.file_mapper(
mapper_id integer not null,
file_name_template varchar);
insert into some_schema.file_mapper (mapper_id, file_name_template)
values
(1, '||v_first_name||''_''||v_last_name||')
(2, '||v_last_name||''_''||v_first_name||')
(3, '||v_last_name||''_''||v_last_name||');
Now the function itself
create or replace function some_schema.some_function(integer)
returns varchar as
$body$
Declare
v_filename_config_id alias for $1;
v_filename varchar;
v_first_name varchar;
v_last_name varchar;
cmd text;
Begin
v_first_name :='Joe';
v_last_name :='Shmoe';
cmd := 'select file_name_template
from some_schema.file_mapper
where mapper_id = '||v_filename_config||'';
execute cmd into v_filename;
raise notice 'checking "%"',v_filename;
return 'done';
end;
$body$
LANGUAGE plpgsql;
Now that I have this. I want to be able to mix and match file name patterns. So for instance I wanted to use mapper_id 3, I would expect a returned file of "Shmoe_Shmoe.csv" if I execute the script:
select from some_schema.some_function(2)
The Problem is whenever I get it to read the "v_filename" variable it will not evaluate and return the values from the function's variables. Originally, I believed it to be a quoting issue(and it probably still is). After messing with the quoting I have gotten about as far the error below:
ERROR: syntax error at or near "_"
LINE 4: ...s/dir/someplace/||v_last_name||'_'||v_firs...
^
QUERY: copy(
select *
from some_schema.some_table)
to '/dir/someplace/||v_last_name||'_'||v_first_name||.csv/;
DELIMITER,
csv HEADER;
CONTEXT: PL/pgSQL function some_schema.some_function(integer) line 27 at EXECUTE statement
As you can tell it is pretty much telling me it is a quoting issue. Is there a way I can get the function to properly evaluate the variable and return the proper file name? Let me know if I am not clear and need to elaborate.

Answer 1

This more or less illustrates the usage of format(). (Slightly reduced wrt the original question):
CREATE TABLE file_mapper
( mapper_id INTEGER NOT NULL PRIMARY KEY
, file_name_template TEXT
);
INSERT INTO file_mapper(mapper_id, file_name_template) VALUES (1,'one'), (2, 'two');
CREATE OR REPLACE FUNCTION dump_the_shit(INTEGER)
RETURNS VARCHAR AS
$body$
DECLARE
v_filename_config_id alias for $1;
v_filename VARCHAR;
name_cmd TEXT;
copy_cmd TEXT;
BEGIN
name_cmd := format( e'select file_name_template
from file_mapper
where mapper_id = %L;', v_filename_config_id );
EXECUTE name_cmd into v_filename;
RAISE NOTICE 'V_filename := %', v_filename;
copy_cmd := format( e'copy(
select *
from %I)
to \'/tmp/%s.csv\'
csv HEADER;' , 'file_mapper' , v_filename);
EXECUTE copy_cmd;
RETURN copy_cmd;
END;
$body$
LANGUAGE plpgsql;
SELECT dump_the_shit(1);
format function description
summary:
use %I for identifiers (tablenames and column names) [ FROM %I ]
use %L for literals such as query constants [ WHERE the_date = %L ]
use %s for ordinary strings [ to \'/tmp/%s.csv\' ]