Re-train a model

Learn how to retrain a machine learning model in ML.NET.

The world and the data around it change at a constant pace. As such, models need to change and update as well. ML.NET provides functionality for re-training models using learned model parameters as a starting point to continually build on previous experience rather than starting from scratch every time.

The following algorithms are re-trainable in ML.NET:

Load pre-trained model

First, load the pre-trained model into your application. To learn more about loading training pipelines and models, see Save and load a trained model.

// Create MLContext
MLContext mlContext = new MLContext();

// Define DataViewSchema of data prep pipeline and trained model
DataViewSchema dataPrepPipelineSchema, modelSchema;

// Load data preparation pipeline
ITransformer dataPrepPipeline = mlContext.Model.Load("", out dataPrepPipelineSchema);

// Load trained model
ITransformer trainedModel = mlContext.Model.Load("", out modelSchema);

Extract pre-trained model parameters

Once the model is loaded, extract the learned model parameters by accessing the Model property of the pre-trained model. The pre-trained model was trained using the linear regression model OnlineGradientDescentTrainer which creates a RegressionPredictionTransformer that outputs LinearRegressionModelParameters. These model parameters contain the learned bias and weights or coefficients of the model. These values will be used as a starting point for the new re-trained model.

// Extract trained model parameters
LinearRegressionModelParameters originalModelParameters =
    ((ISingleFeaturePredictionTransformer<object>)trainedModel).Model as LinearRegressionModelParameters;


The model parameters output depend on the algorithm used. For example OnlineGradientDescentTrainer uses LinearRegressionModelParameters, while LbfgsMaximumEntropyMulticlassTrainer outputs MaximumEntropyModelParameters. When extracting model parameters, cast to the appropriate type.

Re-train model

The process for retraining a model is no different than that of training a model. The only difference is, the Fit method in addition to the data also takes as input the original learned model parameters and uses them as a starting point in the re-training process.

// New Data
HousingData[] housingData = new HousingData[]
    new HousingData
        Size = 850f,
        HistoricalPrices = new float[] { 150000f,175000f,210000f },
        CurrentPrice = 205000f
    new HousingData
        Size = 900f,
        HistoricalPrices = new float[] { 155000f, 190000f, 220000f },
        CurrentPrice = 210000f
    new HousingData
        Size = 550f,
        HistoricalPrices = new float[] { 99000f, 98000f, 130000f },
        CurrentPrice = 180000f

//Load New Data
IDataView newData = mlContext.Data.LoadFromEnumerable<HousingData>(housingData);

// Preprocess Data
IDataView transformedNewData = dataPrepPipeline.Transform(newData);

// Retrain model
RegressionPredictionTransformer<LinearRegressionModelParameters> retrainedModel =
        .Fit(transformedNewData, originalModelParameters);

At this point, you can save your re-trained model and use it in your application. For more information, see the save and load a trained model and make predictions with a trained model guides.

Compare model parameters

How do you know if re-training actually happened? One way would be to compare whether the re-trained model's parameters are different than those of the original model. The code sample below compares the original against the re-trained model weights and outputs them to the console.

// Extract Model Parameters of re-trained model
LinearRegressionModelParameters retrainedModelParameters = retrainedModel.Model as LinearRegressionModelParameters;

// Inspect Change in Weights
var weightDiffs =
        retrainedModelParameters.Weights, (original, retrained) => original - retrained).ToArray();

Console.WriteLine("Original | Retrained | Difference");
for(int i=0;i < weightDiffs.Count();i++)
    Console.WriteLine($"{originalModelParameters.Weights[i]} | {retrainedModelParameters.Weights[i]} | {weightDiffs[i]}");

The table below shows what the output might look like.

Original Retrained Difference
33039.86 56293.76 -23253.9
29099.14 49586.03 -20486.89
28938.38 48609.23 -19670.85
30484.02 53745.43 -23261.41