What is the correct way to structure this generic

Ignoring the fact that this uses the Aurelius Framework, this question is more about how I need to re-tweak the code to make the generic constructor injection work for both type:
< string >
and
< TCustomConnection >
Also ignore the fact that the child objects are in the same unit, I would generally put them in their own, but this just makes it easier to post in a question.

I'm trying use the Factory Method pattern to determine what type of connection it should be making at runtime depending on what object I instantiate. At the moment it is hardcoding the type of link it is expecting on the create.

In the example I want to pass in a TModelDatabaseLink, but want to decide at run time what type of database connection it could be, or whether the database connection comes from a file. Yes I know I could uncomment FFilename and just use this to hold the file name version, but I am always interested in learning a bit more.

unit Model.Database.Connection;

interface

uses
  System.Classes,
  Data.DB,
  Aurelius.Drivers.Interfaces,
  Aurelius.Engine.DatabaseManager;

type
  TModelDatabaseLink<T> = class
  private
    //FFilename: string;
    FConnection: T;
    FOwnsConnection: boolean;
    OwnedComponent: TComponent;
  end;

  TModelDatabaseConnection = class abstract
  private
    FDatabaseLink: TModelDatabaseLink<TCustomConnection>;
    FDatabaseManager: TDatabaseManager;
    FConnection: IDBConnection;
    function CreateConnection: IDBConnection; virtual; abstract;
    procedure CreateDatabaseManager;
  public
    constructor Create(ADatabaseLink: TModelDatabaseLink<TCustomConnection>);
    destructor Destroy; override;

    property Connection: IDBConnection read FConnection;
  end;

  TSQLLiteConnection = class(TModelDatabaseConnection)
  private
    function CreateConnection: IDBConnection; override;
  end;

  TFireDacConnection = class(TModelDatabaseConnection)
  private
    function CreateConnection: IDBConnection; override;
  end;

implementation

uses
  System.SysUtils,

  Aurelius.Drivers.Base,
  Aurelius.Drivers.SQLite,
  Aurelius.Drivers.FireDac,

  FireDAC.Stan.Intf, FireDAC.Stan.Option,
  FireDAC.Stan.Error, FireDAC.UI.Intf, FireDAC.Phys.Intf, FireDAC.Stan.Def,
  FireDAC.Stan.Pool, FireDAC.Stan.Async, FireDAC.Phys, FireDAC.VCLUI.Wait,
  FireDAC.Comp.Client;

{ TModelDatabaseConnection }

constructor TModelDatabaseConnection.Create(ADatabaseLink: TModelDatabaseLink<TCustomConnection>);
begin
  FDatabaseLink := ADatabaseLink;
  FConnection := CreateConnection;
  if Assigned(FConnection) then
    CreateDatabaseManager
  else
    raise Exception.Create('Failed to open database');
end;

procedure TModelDatabaseConnection.CreateDatabaseManager;
begin
  FDatabaseManager := TDatabaseManager.Create(FConnection);
end;

destructor TModelDatabaseConnection.Destroy;
begin
  FDatabaseManager.Free;
  FDatabaseLink.Free;
  inherited Destroy;
end;

{ TSQLLiteConnection }

function TSQLLiteConnection.CreateConnection: IDBConnection;
var
  LFilename: String;
  LAdapter: TSQLiteNativeConnectionAdapter;
begin
  //LFileName := FDatabaseLink.FConnection;                     << needs to be type string
  LAdapter := TSQLiteNativeConnectionAdapter.Create(LFilename);
  LAdapter.DisableForeignKeys;
  Result := LAdapter;
end;

{ TFireDacConnection }

function TFireDacConnection.CreateConnection: IDBConnection;
var
  LAdapter: TFireDacConnectionAdapter;
begin
  if Assigned(FDatabaseLink.OwnedComponent) then
    LAdapter := TFireDacConnectionAdapter.Create(FDatabaseLink.FConnection as TFDConnection, FDatabaseLink.OwnedComponent)
  else
    LAdapter := TFireDacConnectionAdapter.Create(FDatabaseLink.FConnection as TFDConnection, FDatabaseLink.FOwnsConnection);

  Result := LAdapter;
end;

end.

One other thing I would like to do if possible, is to change two creations:

LAdapter := TSQLiteNativeConnectionAdapter.Create(LFilename)

LAdapter := TFireDacConnectionAdapter.Create(FDatabaseLink.FConnection as TFDConnection, FDatabaseLink.OwnedComponent)

to use an abstract "GetAdapterClass" type function in the parent TModelDatabaseConnection and just declare the class of adapter in the child to do something like:

LAdapter := GetAdapterClass.Create...

An example of an adapter declaration is

TFireDacConnectionAdapter = class(TDriverConnectionAdapter<TFDConnection>, IDBConnection)

I have done this before when I wrote an abstract layer in case I need to replace Aurelius in my applications. I think the best way to deal with what you want to do is to use interfaces.

I am copying here some parts of my code with adjustments:

  TServerType = (stLocal, stFireDac);

  IDatabase = interface
    function getDatabaseType: TServerType;
    property DatabaseType: TServerType read getDatabaseType;
  end;

  IAurelius = interface (IDatabase)
    ['{990BB776-2E70-4140-B118-BEFF61FDBDAF}']
    function getDatabaseConnection: IDBConnection;
    function getDatabaseManager: TDatabaseManager;
    property DatabaseConnection: IDBConnection read getDatabaseConnection;
    property DatabaseManager: TDatabaseManager read getDatabaseManager;
  end;

  IAureliusLocal = interface (IAurelius)
    ['{9F705CC4-6E3B-4706-B54A-F0649CED3A8D}']
    function getDatabasePath: string;
    property DatabasePath: string read getDatabasePath;
  end;

  IAureliusFireDac = interface (IAurelius)
    // I use this for a memory database but the logic is the same for FireDAC. You need to add the required properties 
  end;

  TAurelius = class (TInterfacedObject, IAurelius)
  private
    fServerType: TServerType;
    fDatabaseConnection: IDBConnection;
    fDatabaseManager: TDatabaseManager;
    function getDatabaseConnection: IDBConnection;
    function getDatabaseManager: TDatabaseManager;
  public
    constructor Create (const serverType: TServerType);
  end;

  TAureliusLocal = class (TAurelius, IAureliusLocal)
  private
    fDatabasePath: string;
    function getDatabasePath: string;
  public
    constructor Create (const databasePath: string);
  end;

  TAureliusFireDac = class (TAurelius, IAureliusFireDac)
  public 
    constructor Create (const aConnection: TFDConenction); <-- or whatever parameters you need here to initalise the FireDac connection
  end;

I am going to skip the code for all the getXXX functions here.

The constructors are these:

constructor TAurelius.Create(const serverType: TServerType);
begin
  inherited Create;
  fServerType:=serverType;
end;

constructor TAureliusLocal.Create (const databasePath: string);
const
  databaseFilename = 'test.sqlite';
begin
  inherited Create(stLocal);
  fDatabasePath:=Trim(databasePath);
   try
    fDatabaseConnection:=
    TSQLiteNativeConnectionAdapter.Create(
      TPath.Combine(fDatabasePath, databaseFilename));
   except
    raise Exception.Create('stLocal database can''t be created');
   end;
end;

constructor TAureliusFireDac.Create (const aConnection: TFDConenction);
begin
  inherited Create(stFireDac);
  // <-- here you initialise the connection like before but for FireDac
end;

Now, when you want to create an Aurelius database you use the following functions:

function createAureliusDatabase (const serverType: TServerType): IAurelius;
begin
  case serverType of
    stLocal: result:=TAureliusLocal.Create(path);
    stFireDac: result:=TAureliusFireDac.Create(....);
  end;
end;

...and you simply call it like this:

var 
  currentDatabase: IAurelius;
begin
  currentDatabase:=createAureliusDatabase(stLocal,'c:\....');
end;

A better way to deal with the creation of the database is to use overloaded functions with different parameters or anonymous methods to avoid the case-end branch.