.. _guide-training-edge-classification:

5.2 Edge Classification/Regression
---------------------------------------------

Sometimes you wish to predict the attributes on the edges of the graph,
or even whether an edge exists or not between two given nodes. In that
case, you would like to have an *edge classification/regression* model.

Here we generate a random graph for edge prediction as a demonstration.

.. code:: ipython3

    src = np.random.randint(0, 100, 500)
    dst = np.random.randint(0, 100, 500)
    # make it symmetric
    edge_pred_graph = dgl.graph((np.concatenate([src, dst]), np.concatenate([dst, src])))
    # synthetic node and edge features, as well as edge labels
    edge_pred_graph.ndata['feature'] = torch.randn(100, 10)
    edge_pred_graph.edata['feature'] = torch.randn(1000, 10)
    edge_pred_graph.edata['label'] = torch.randn(1000)
    # synthetic train-validation-test splits
    edge_pred_graph.edata['train_mask'] = torch.zeros(1000, dtype=torch.bool).bernoulli(0.6)

Overview
~~~~~~~~

From the previous section you have learned how to do node classification
with a multilayer GNN. The same technique can be applied for computing a
hidden representation of any node. The prediction on edges can then be
derived from the representation of their incident nodes.

The most common case of computing the prediction on an edge is to
express it as a parameterized function of the representation of its
incident nodes, and optionally the features on the edge itself.

Model Implementation Difference from Node Classification
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Assuming that you compute the node representation with the model from
the previous section, you only need to write another component that
computes the edge prediction with the
:meth:`~dgl.DGLHeteroGraph.apply_edges` method.

For instance, if you would like to compute a score for each edge for
edge regression, the following code computes the dot product of incident
node representations on each edge.

.. code:: python

    import dgl.function as fn
    class DotProductPredictor(nn.Module):
        def forward(self, graph, h):
            # h contains the node representations computed from the GNN above.
            with graph.local_scope():
                graph.ndata['h'] = h
                graph.apply_edges(fn.u_dot_v('h', 'h', 'score'))
                return graph.edata['score']

One can also write a prediction function that predicts a vector for each
edge with an MLP. Such vector can be used in further downstream tasks,
e.g. as logits of a categorical distribution.

.. code:: python

    class MLPPredictor(nn.Module):
        def __init__(self, in_features, out_classes):
            super().__init__()
            self.W = nn.Linear(in_features * 2, out_classes)
    
        def apply_edges(self, edges):
            h_u = edges.src['h']
            h_v = edges.dst['h']
            score = self.W(torch.cat([h_u, h_v], 1))
            return {'score': score}
    
        def forward(self, graph, h):
            # h contains the node representations computed from the GNN above.
            with graph.local_scope():
                graph.ndata['h'] = h
                graph.apply_edges(self.apply_edges)
                return graph.edata['score']

Training loop
~~~~~~~~~~~~~

Given the node representation computation model and an edge predictor
model, we can easily write a full-graph training loop where we compute
the prediction on all edges.

The following example takes ``SAGE`` in the previous section as the node
representation computation model and ``DotPredictor`` as an edge
predictor model.

.. code:: python

    class Model(nn.Module):
        def __init__(self, in_features, hidden_features, out_features):
            super().__init__()
            self.sage = SAGE(in_features, hidden_features, out_features)
            self.pred = DotProductPredictor()
        def forward(self, g, x):
            h = self.sage(g, x)
            return self.pred(g, h)

In this example, we also assume that the training/validation/test edge
sets are identified by boolean masks on edges. This example also does
not include early stopping and model saving.

.. code:: python

    node_features = edge_pred_graph.ndata['feature']
    edge_label = edge_pred_graph.edata['label']
    train_mask = edge_pred_graph.edata['train_mask']
    model = Model(10, 20, 5)
    opt = torch.optim.Adam(model.parameters())
    for epoch in range(10):
        pred = model(edge_pred_graph, node_features)
        loss = ((pred[train_mask] - edge_label[train_mask]) ** 2).mean()
        opt.zero_grad()
        loss.backward()
        opt.step()
        print(loss.item())


Heterogeneous graph
~~~~~~~~~~~~~~~~~~~

Edge classification on heterogeneous graphs is not very different from
that on homogeneous graphs. If you wish to perform edge classification
on one edge type, you only need to compute the node representation for
all node types, and predict on that edge type with
:meth:`~dgl.DGLHeteroGraph.apply_edges` method.

For example, to make ``DotProductPredictor`` work on one edge type of a
heterogeneous graph, you only need to specify the edge type in
``apply_edges`` method.

.. code:: python

    class HeteroDotProductPredictor(nn.Module):
        def forward(self, graph, h, etype):
            # h contains the node representations for each edge type computed from
            # the GNN above.
            with graph.local_scope():
                graph.ndata['h'] = h   # assigns 'h' of all node types in one shot
                graph.apply_edges(fn.u_dot_v('h', 'h', 'score'), etype=etype)
                return graph.edges[etype].data['score']

You can similarly write a ``HeteroMLPPredictor``.

.. code:: python

    class MLPPredictor(nn.Module):
        def __init__(self, in_features, out_classes):
            super().__init__()
            self.W = nn.Linear(in_features * 2, out_classes)
    
        def apply_edges(self, edges):
            h_u = edges.src['h']
            h_v = edges.dst['h']
            score = self.W(torch.cat([h_u, h_v], 1))
            return {'score': score}
    
        def forward(self, graph, h, etype):
            # h contains the node representations computed from the GNN above.
            with graph.local_scope():
                graph.ndata['h'] = h   # assigns 'h' of all node types in one shot
                graph.apply_edges(self.apply_edges, etype=etype)
                return graph.edges[etype].data['score']

The end-to-end model that predicts a score for each edge on a single
edge type will look like this:

.. code:: python

    class Model(nn.Module):
        def __init__(self, in_features, hidden_features, out_features, rel_names):
            super().__init__()
            self.sage = RGCN(in_features, hidden_features, out_features, rel_names)
            self.pred = HeteroDotProductPredictor()
        def forward(self, g, x, etype):
            h = self.sage(g, x)
            return self.pred(g, h, etype)

Using the model simply involves feeding the model a dictionary of node
types and features.

.. code:: python

    model = Model(10, 20, 5, hetero_graph.etypes)
    user_feats = hetero_graph.nodes['user'].data['feature']
    item_feats = hetero_graph.nodes['item'].data['feature']
    label = hetero_graph.edges['click'].data['label']
    train_mask = hetero_graph.edges['click'].data['train_mask']
    node_features = {'user': user_feats, 'item': item_feats}

Then the training loop looks almost the same as that in homogeneous
graph. For instance, if you wish to predict the edge labels on edge type
``click``, then you can simply do

.. code:: python

    opt = torch.optim.Adam(model.parameters())
    for epoch in range(10):
        pred = model(hetero_graph, node_features, 'click')
        loss = ((pred[train_mask] - label[train_mask]) ** 2).mean()
        opt.zero_grad()
        loss.backward()
        opt.step()
        print(loss.item())


Predicting Edge Type of an Existing Edge on a Heterogeneous Graph
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes you may want to predict which type an existing edge belongs
to.

For instance, given the heterogeneous graph above, your task is given an
edge connecting a user and an item, predict whether the user would
``click`` or ``dislike`` an item.

This is a simplified version of rating prediction, which is common in
recommendation literature.

You can use a heterogeneous graph convolution network to obtain the node
representations. For instance, you can still use the RGCN above for this
purpose.

To predict the type of an edge, you can simply repurpose the
``HeteroDotProductPredictor`` above so that it takes in another graph
with only one edge type that “merges” all the edge types to be
predicted, and emits the score of each type for every edge.

In the example here, you will need a graph that has two node types
``user`` and ``item``, and one single edge type that “merges” all the
edge types from ``user`` and ``item``, i.e. ``like`` and ``dislike``.
This can be conveniently created using
:meth:`relation slicing <dgl.DGLHeteroGraph.__getitem__>`.

.. code:: python

    dec_graph = hetero_graph['user', :, 'item']

Since the statement above also returns the original edge types as a
feature named ``dgl.ETYPE``, we can use that as labels.

.. code:: python

    edge_label = dec_graph.edata[dgl.ETYPE]

Given the graph above as input to the edge type predictor module, you
can write your predictor module as follows.

.. code:: python

    class HeteroMLPPredictor(nn.Module):
        def __init__(self, in_dims, n_classes):
            super().__init__()
            self.W = nn.Linear(in_dims * 2, n_classes)
    
        def apply_edges(self, edges):
            x = torch.cat([edges.src['h'], edges.dst['h']], 1)
            y = self.W(x)
            return {'score': y}
    
        def forward(self, graph, h):
            # h contains the node representations for each edge type computed from
            # the GNN above.
            with graph.local_scope():
                graph.ndata['h'] = h   # assigns 'h' of all node types in one shot
                graph.apply_edges(self.apply_edges)
                return graph.edata['score']

The model that combines the node representation module and the edge type
predictor module is the following:

.. code:: python

    class Model(nn.Module):
        def __init__(self, in_features, hidden_features, out_features, rel_names):
            super().__init__()
            self.sage = RGCN(in_features, hidden_features, out_features, rel_names)
            self.pred = HeteroMLPPredictor(out_features, len(rel_names))
        def forward(self, g, x, dec_graph):
            h = self.sage(g, x)
            return self.pred(dec_graph, h)

The training loop then simply be the following:

.. code:: python

    model = Model(10, 20, 5, hetero_graph.etypes)
    user_feats = hetero_graph.nodes['user'].data['feature']
    item_feats = hetero_graph.nodes['item'].data['feature']
    node_features = {'user': user_feats, 'item': item_feats}
    
    opt = torch.optim.Adam(model.parameters())
    for epoch in range(10):
        logits = model(hetero_graph, node_features, dec_graph)
        loss = F.cross_entropy(logits, edge_label)
        opt.zero_grad()
        loss.backward()
        opt.step()
        print(loss.item())


DGL provides `Graph Convolutional Matrix
Completion <https://github.com/dmlc/dgl/tree/master/examples/pytorch/gcmc>`__
as an example of rating prediction, which is formulated by predicting
the type of an existing edge on a heterogeneous graph. The node
representation module in the `model implementation
file <https://github.com/dmlc/dgl/tree/master/examples/pytorch/gcmc>`__
is called ``GCMCLayer``. The edge type predictor module is called
``BiDecoder``. Both of them are more complicated than the setting
described here.


