Delphi2005 - Using TADOQuery Results in another method

Question

Because of code redundancy I generalized the usual ADOQuery handling into a method of its own, with the method returning the _RecordSet of the query. Now, when I check the query results inside that method, they check out alright. But the returned _RecordSet seems to be empty or nil (Exception: EOleException 'Item cannot be found in the collection to the corresponding name or ordinal.') From what I gathered, I seem to need to return a clone of the query's _RecordSet. So far I have tried using

res := qr.Recordset.Clone(adLockUnspecified); Result := res;,

TADOQuery.RecordSet._xClone() method

unsuccessfully, as well as THESE methods, none of which worked (several components not recognized, version differences I'm guessing).

The code I use for querying:

function queryDb(const s: WideString; const dc: boolean = true): _RecordSet;
var
  ds: String;
  conn: TADOConnection;
  qr: TADOQuery;
  res: _RecordSet;
begin
  Result := nil;
  conn := TADOConnection.Create(nil);
  qr := TADOQuery.Create(nil);
  try
    ds := 'Provider=Microsoft.Jet.OLEDB.4.0;Data Source='
          + ExtractFilePath(Application.ExeName)
          + 'gdkp.mdb;Persist Security Info=False';
    conn.ConnectionString := ds;
    conn.LoginPrompt := false;
    conn.Connected := true;
    qr.Connection := conn;
    if(dc = true)then begin
      qr.DisableControls
    end;
    qr.SQL.Add(s);
    qr.Open;
    Result := qr.Recordset;
  finally
    conn.Free;
    qr.Free;
  end;
end;

---

Has someone overcome this problem before and knows a suitable answer or can direct me towards helpful content?

Answer 1

I don't know which of your crazy antics is more crazy:

1. Setting up and connecting to a database inside a function and doing your queries inside this function, connecting to the database EACH TIME YOU QUERY, is going to be insanely slow.

2. ADO Query objects should have a longer lifetime than the insides of a function. This is a code smell, a severe one.

3. Keep the recordset inside the query where it belongs.

4. Normal people create TDataModules and leave their objects around for the lifetime of their app, and requery using this code:

   query.Active := false; query.Active := true;

Watch your app speed up when it doesn't have to connect to the database, create and tear down a connection and a query, each time you want some data.

What on earth are you doing with the _recordsets youre getting back? Did you learn ADO in C# and you're trying to use C# ADO.net idioms in Delphi? Don't. Stop doing that.

Answer 2

Your returned RecordSet is nil/empty because you are closing it in the finally statement. I'm guessing this is why you have tried cloning the RecordSet before returning? Cloning it creates another pointer to the same RecordSet so closing the RecordSet will also cause your clone to return nil/empty.

As Ben has suggested, there are better ways to do this like using the TDataModules, but here's my way:

I tend to write a function for each data retrieval or update/insert operation. Each function is passed an ADO connection as a parameter and then the matter of running the query/stored proc is contained within the function and any data is returned as a var parameter or as the return value. I keep a single ADO connection which is passed around to each function as needed.

e.g (pseudo code because I've not done Delphi for a while)

function GetAllUsers(adoConnection : TAdoConnection) : TStringList    
begin
  result.clear;
  if (adoConnection <> nil) then
  begin
     ..
     ..
     // have already set up an ADOStoredProc called storedProc in this
     // part of the code block
     ..
    adoRec := storedProc.Execute();
    while not (adoRec.eof) do              // loop through record set and get our data
    begin
      result.Add(adoGetFieldStr(adoRec,'USERNAME'));    
      adoRec.MoveNext;
    end;
    adoRec.Free;
  end;
end

Hope that helps