Object Relational Mapping

Instructions

You are implementing an ORM (Object Relational Mapping) system over a database which has been provided by another team.

The database is capable of handling a single transaction at one time.

No logging or other error handling is required at this stage.

Note that, internally, the database transitions through a number of state: Closed, TransactionStarted, DataWritten, Invalid, Closed. You can rely on the fact that the database is in a Closed state at the start of each exercise.

The database has the following instance methods:

• Database.BeginTransaction() starts a transaction on the database. If this is called when the database is not in a Closed state then an exception is thrown. If successful the internal state of the database will change to TransactionStarted.
• Database.Write(string data) writes data to the database within the transaction. If it receives bad data an exception will be thrown. An attempt to call this method without BeginTransaction() having been called will cause an exception to be thrown. If successful the internal state of the database will change to DataWritten.
• Database.EndTransaction() commits the transaction to the database. It may throw an exception if it can't close the transaction or if Database.BeginTransaction() had not been called.
• A call toDatabase.Dispose() will clean up the database if an exception is thrown during a transaction. This will change the state of the database to Closed.

1. Begin a transaction

Implement Orm.Begin() to start a transaction on the database. If the database does not start with an internal state of State.Closed then it throws an InvalidOperationException.

var orm = new Orm(new Database());
orm.Begin();
// => database has an internal state of State.TransactionStarted

2. Write some data to the database

Implement Orm.Write() to write some data to the database. If the database does not start with an internal state of State.TransactionStarted or bad data is written then an InvalidOperationException is thrown. If the write fails then the Orm must clean up the database.

var orm = new Orm(new Database());
orm.Begin();
orm.Write("some data");
// => database has an internal state of State.DataWritten
orm.Write("bad write");
// => database has an internal state of State.Closed

3. Commit previously written data to the database

Implement Orm.Commit() to commit the data. If the commit fails then clean up the database. If the database does not start with an internal state of State.DataWritten then an InvalidOperationException is thrown.

var orm = new Orm(new Database());
orm.Begin();
orm.Write("some data");
orm.Commit();
// => database has an internal state of State.Closed
orm.Begin();
orm.Write("bad commit");
orm.Commit();
// => database has an internal state of State.Closed

4. Ensure that the database is cleaned up correctly if the ORM has to close part way through a transaction.

Implement the IDisposable interface on the Orm class. The call is guaranteed to succeed.

var db = new Database();
var orm = new Orm(db);
orm.Begin();
orm.Write("some data");
orm.Dispose();
// => database has an internal state of State.Closed