How do I use an ID throughout a Delphi app - delphi

Question

I have a MySql database which has a users table and all the other tables are connected to the users table.
If we log in to an user that has id = 1. We want to make all the requests to mysql using the user_id = 1, so it'll be displayed only the data from this specific user.
I have a login screen in my Delphi app where the user can type his username and password. After a successfully login I want to take thie user_id to use and throughout this application to make the calls. How should I do it?
After the login in successfully, should I sotre this ID in Delphi somehow and access it whenever we want to make some call? If so, how do I save this id in Delphi so I can use it in several forms?

Answer 1

A way to do this is to add a DataModule to your project and make the UserID a
field of this.
So, using Delphi's default naming, you would have
type
TDataModule1 = class(TDataModule)
[...]
public
property UserID : Integer read FUserID;
end;
[...]
var
DataModule1 : TDataModule1;
Then, add to TDataModule1 the db-access components your project uses, e.g. a Query
component to access the Users table where user details are stored and a method to
try to login in a given user, and also a db-connection component of the type your db-access components use. So you could add a method like this:
function TDataModule1.LogInUser(const UserName : String) : Boolean;
begin
try
if qUsers.Active then
qUsers.Close;
FUserID := -1; // deliberately set an invalid UserID
Assert(Trim(UserName) <> '')); // raise exception if UserName is blank
// qUsers Sql.Text property would be something like 'select * from users where username = :username'
qUsers.ParamByName(UserName).Value := UserName;
qUsers.Open;
if qUsers.Locate('UserName', UserName, []) then
FUserID := qUsers.FieldByName('UserID').AsInteger;
finally
Result := FUserID >= 0; // assuming zero is a valid UserID
end;
end;
and save the module under the name, say, of DataModule1u. Notice that this method omits any reference to the user's password: This is because if you leave your db connection component's LoginPrompt property at the default True value, it will pop up a prompt for the user's password when the LogInUser method executes.
Once you've got that far, go to your Login form's unit and edit it to USE DataModule1u.
Then you can add code to it to use the LogInUser method like so:
if DataModule1.LogInUser(UserName) then begin
Caption := 'User: ' + IntToStr(DataModule1.UserID);
// do something else with DataModule1.UserID's value
end;
Obviously you could (and usually should) add all your db-access components to the datamodule
and USE it in your forms which do db-access.
Hope this is all clear, if not then ask.