In the previous posting, we learned how to train and evaluate a matrix factorization (MF) model with the fast.ai package. Nowadays, with sheer developments in relevant fields, neural extensions of MF such as NeuMF (He et al. 2017) and Deep MF (Xue et al. 2017) became very popular. In this posting, let’s have a look at a very simple variant of MF using multilayer perceptron.

Data Import & Preparation

These steps are identical to preparing for MF. If you haven’t yet, please have a look at this previous posting for importing and preparing the data.

Creating and training a neural collaborative filtering model

We use the same collab_learner() function that was used for implementing the MF model. Parameters that should be changed to implement a neural collaborative filtering model are use_nn and layers. Setting use_nn to True implements a neural network. Recall that the MF model had only embedding layers for users and items. layers parameter lets us define the architecture of the neural network. In specific, we can designate the numbers of nodes in hidden layers. Here, let’s set it to [30, 30] - by doing so, we are generating a neural network with two hidden layers having 30 nodes each.

learn = collab_learner(databunch, n_factors=50, y_range=(0, 5), use_nn = True, layers = [30, 30])
learn.model

You can see that the resulting model has three additional Linear() layers. The final one is the output layer and the first two are the hidden layers that we have configured. Note that out_features for the two layers are set to 30.

EmbeddingNN(
  (embeds): ModuleList(
    (0): Embedding(944, 74)
    (1): Embedding(1625, 101)
  )
  (emb_drop): Dropout(p=0.0, inplace=False)
  (bn_cont): BatchNorm1d(0, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
  (layers): Sequential(
    (0): Linear(in_features=175, out_features=30, bias=True)
    (1): ReLU(inplace=True)
    (2): BatchNorm1d(30, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
    (3): Linear(in_features=30, out_features=30, bias=True)
    (4): ReLU(inplace=True)
    (5): BatchNorm1d(30, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
    (6): Linear(in_features=30, out_features=1, bias=True)
  )

To train the model with the given data, we use fit() function. We train 5 epochs here.

learn.fit(5)

epoch train_loss  valid_loss  time
0 0.969993  0.921394  00:07
1 0.907009  0.894607  00:06
2 0.866429  0.886609  00:06
3 0.863333  0.877121  00:06
4 0.822304  0.874548  00:06

To evaluate the model on the test data, we can use get_preds() function to get model predictions and convert them into a NumPy array.

from sklearn.metrics import *

y_pred = learn.get_preds(ds_type = DatasetType.Test)[0].numpy()
print(mean_absolute_error(test_df["rating"], y_pred))

The model shows slightly improved performance compared to MF. You can experiment on other configurations, e.g., making the model deeper by adding more layers or wider by adding nodes to the hidden layers.

0.7473991450190545

References

• Collaborative filtering tutorial. (https://docs.fast.ai/tutorial.collab)
• Collaborative filtering using fastai. (https://towardsdatascience.com/collaborative-filtering-using-fastai-a2ec5a2a4049)
• He, X., Liao, L., Zhang, H., Nie, L., Hu, X., & Chua, T. S. (2017, April). Neural collaborative filtering. In Proceedings of the 26th international conference on world wide web (pp. 173-182).
• Xue, H. J., Dai, X., Zhang, J., Huang, S., & Chen, J. (2017, August). Deep Matrix Factorization Models for Recommender Systems. In IJCAI (Vol. 17, pp. 3203-3209).