adds Database class to ObjectRelationalMapping.cs

It was very confusing for me, to do this exercise as I was not able to check how the Database class looks like. I could not check the logic, whether I have to impelement some of the part, which were already in Database class. In the end, the exercise was easy, but I had to download it locally and check the tests.

I suggest to add there how the Database is implemented.

Author: Jazzman94

exercises/concept/object-relational-mapping/ObjectRelationalMapping.cs

@@ -22,3 +22,59 @@ public void Commit()
         throw new NotImplementedException($"Please implement the Orm.Commit() method");
     }
 }
+
+// **** please do not modify the Database class ****
+public class Database : IDisposable
+{
+    public enum State { TransactionStarted, DataWritten, Invalid, Closed }
+
+    public State DbState { get; private set; } = State.Closed;
+    public string lastData = string.Empty;
+
+    public void BeginTransaction()
+    {
+        if (DbState != State.Closed)
+        {
+            throw new InvalidOperationException();
+        }
+        DbState = State.TransactionStarted;
+    }
+
+    public void Write(string data)
+    {
+        if (DbState != State.TransactionStarted)
+        {
+            throw new InvalidOperationException();
+        }
+        // this does something significant with the db transaction object
+        lastData = data;
+        if (data == "bad write")
+        {
+            DbState = State.Invalid;
+            throw new InvalidOperationException();
+        }
+
+        DbState = State.DataWritten;
+    }
+
+    public void EndTransaction()
+    {
+        if (DbState != State.DataWritten && DbState != State.TransactionStarted)
+        {
+            throw new InvalidOperationException();
+        }
+        // this does something significant to end the db transaction object
+        if (lastData == "bad commit")
+        {
+            DbState = State.Invalid;
+            throw new InvalidOperationException();
+        }
+
+        DbState = State.Closed;
+    }
+
+    public void Dispose()
+    {
+        DbState = State.Closed;
+    }
+}