如何从字段获取列名

Question

I have searched the web and looked in the source code of the postgresql-simple package.

The results of a query (or query_) are of type class [QueryResults]. It is easy to get the contents of the fields, but can anyone show some example code for getting the column/field names? The Field class from PostgreSQL.Simple.FromField has a name attribute returning a ByteString with the column name. But I am simply unable to get from this simple example listing the contents:

  conn <- connect con
  xs <- query_ conn "select pk,tfrom from feedback"
  forM_ xs $ \(field1,field2) ->
    putStrLn $ T.unpack field2 ++ " has key " ++ show (field1 :: Int)

to something that also lists the name of the columns (ie pk and tfrom).

Getting the field/column names is very useful when doing select * from a (say, human-readable diagnostics, statistics) stored procedure (or table) so we can easily print the results without changing the code when the table or stored procedure changes.

Edited after answer was provided: Here is the working code with the first suggestion (B8 is ByteString):

  putStrLn $ " is names " ++ (B8.toString (fromJust (name field2))) ++ " " ++ (B8.toString (fromJust (name field1)))

Now, there still remains the problem of getting both the data and field names when using the first suggestion as the code will fix the contents to type Field - which does not contain the data itself, just metadata. The second suggestion fixes that, now the working code looks like this:

  forM_ xs $ \((data1,field1),(data2,field2)) ->
    putStrLn $ " " ++ (B8.toString (fromJust (F.name field1))) ++ " " ++ (B8.toString (fromJust (F.name field2))) ++ " " ++ (T.unpack data2) ++ " " ++ (show (data1 :: Int))

I hope this proves useful for someone else as well.

Answer 1

Not a proper answer I'm afraid, but too long for a comment.

If you look at the library's source, you'll see the following function in Simple/Internal.hs

name Field{..} = unsafePerformIO (PQ.fname result column)

The PQ is a reference to libpq (the standard PostgreSQL client library) and fname gets you a fieldname given a resultset and column index. So, clearly it's available.

Now, I don't think this function is exported, and I don't know if there's a way around that since I'm a Haskell newbie. Clearly you could patch the source easily enough.

However, I don't think you'll be able to have a dynamic resultset even with the fieldname available without casting all the query column-types to text (or something similar). Otherwise postgresql-simple will return them as Int or Date or some such.

Answer 2

I don't have a Postgres instance around to check this at the moment, but you could try introducing instance FromField Field where fromField f = return . const f instance FromField Field where fromField f = return . const f and then use name to get the name of your fields out. You might even more generally want

instance FromField a => FromField (a, Field) where
  fromField f bs = (,f) <$> fromField f bs

Unless I'm pretty mistaken on how this library works, I'm pretty surprised those instances aren't in there.