Adding the initial prototype of a DatabaseLoader

[tannergooding]

This adds the initial barebones prototype for DatabaseLoader to the Microsot.ML.Experimental project.

[tannergooding]

CC. @eerhardt

[eerhardt]

Instead of doing this, we should change the test to use the public APIs. Basically new DatabaseLoader => mlContext.Data.CreateDatabaseLoader

[tannergooding]

Fixed.

[eerhardt]

Do we still need this?

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In reply to: 308308500 [](ancestors = 308308500)

[eerhardt]

I think this is the only comment I have left on this PR. If this is resolved, and all the builds pass, I think we will be ready to merge this.

[tannergooding]

Yes. (sorry forgot to update the comment)

This is still needed for converting from the DbType to the InternalKind to the public DataKind.

[eerhardt]

We will either need to disable this test, or change it so it doesn't rely on machine state/configuration.

[tannergooding]

I created a MockDbConnection, MockDbCommand, and MockDbDataReader class which wrap a TextLoader. I would guess this needs some cleanup to support more testing scenarios and to match w/e conventions ML.NET is currently using.

[CESARDELATORRE]

In addition to that way using a Columns array, when possible, can we have another test (and implementation if not ready yet) that is using a convenient data-model class?

[CESARDELATORRE]

Once we have this foundational API we also need to add it to the MLContext catalog, so having convenient methods such as:

mlContext.Data.LoadFromDbDataReader<ModelInputData>(() => command.ExecuteReader());

or

IDataView trainingDataView = mlContext.Data.LoadFromDbTable<ModelInputData, SqlConnection>(connString: myConnString, 
                                                                                                 tableName: "TrainingDataTable"); 

[CESARDELATORRE]

I put a couple of comments but those asks don't need to be implemented before merging this first version. It could come later. :)

[eerhardt]

LGTM. Let's get this in and iterate on it.