it to infer new triples in a testing graph with unseen entities, e.g.,
(H,husband_of,K) in Fig. 1b. GraIL [31] is a typical and recent
method of this kind, with the scalability and rule expressiveness
improved by general graph topological features and a relation-
aware graph neural network for message passing and learning
over local subgraphs (see more details in Section
II-B
). GraIL
and its follow-up methods [7], [20] have shown the feasibility
of using message passing over subgraphs for inductive KGC
with promising results achieved on several benchmarks.
However, these methods mostly assume that all the relations
in the testing stage are seen with embeddings learned, which
is often violated in real-world evolving KGs, especially those
constructed via open information extraction systems e.g. NELL
[23] and those that are publicly editable e.g. Wikidata [35].
Moreover, they often pass messages directly over entities with
the patterns on relations not fully utilized. Although some
strategies have been developed to strengthen the role of rela-
tions, there is still much space to explore. For convenience, we
call the inductive KGC cases with only unseen entities during
testing investigated in these works as
partially inductive KGC
,
and call those more realistic and more challenging cases with
both unseen entities and unseen relations during testing as
fully
inductive KGC1
. Fig. 1 provides clear examples on these two
cases. Currently, there have been few methods specifically
developed for the latter. The only one we know is MaKEr [10]
which also relies on the graph structure for prediction.
In this study, we aim to address fully inductive KGC with
a method named RMPI, which includes a relation oriented
message passing network for subgraph reasoning. It first
transforms a triple’s surrounding subgraph in the original KG
into a new relation view graph, where inter-relation features
are more straightforwardly represented. It then learns the
embedding of an unseen relation from the relational subgraph,
by the relational message passing network, where novel graph
pruning and neighborhood attention techniques are developed
for both efficiency and effectiveness, with new neighborhood
aggregations being used for addressing the issue of empty
subgraphs. As an example, for an unseen relation spouse_of in
Fig. 1c, its embedding for predicting the triple with Hand K
can be obtained by aggregating the embeddings of neighboring
relations husband_of,father_of and son_of. Furthermore, RMPI
allows the injection of the KG’s ontological schema, which is
quite common (see KGs like DBpedia [3] and NELL [23]) and
contains complementary relation semantics, for more robust
reasoning on fully inductive KGC. In summary, our main
contributions are the following:
•
We are among the earliest to investigate fully inductive
KGC, considering both subgraph structures and ontologi-
cal schemas.
1
The term fully inductive has been used in some inductive KGC works that
only consider unseen entities [1], [31], meaning the sets of entities seen during
training and testing are disjoint, as against the other semi inductive case where
unseen entities have to be connected to trained seen entities. In our paper,
we name such a fully inductive setting with only unseen entities as
partially
inductive, to distinguish it from the fully inductive setting we investigated.
•
We have proposed a robust KGC model named RMPI
with novel techniques for effective relational message
passing and subgraph reasoning, supporting both partially
and fully inductive KGC.
•
Extensive experiments have been conducted on
4
newly-
constructed benchmarks and
14
public benchmarks from
different KGs. RMPI and its variants often outperform
the baselines including the state-of-the-art methods.
II. PRELIMINARY
We begin by first formally defining the task and notations,
and then briefly introducing the background of subgraph-based
inductive reasoning used in existing works.
A. Problem Formulation
A KG is often denoted as
G={E,R,T }
, where
E
is a set
of entities,
R
is a set of relations, and
T={(h, r, t)|h, t ∈
E;r∈ R}
is a set of relational facts in form of RDF
2
triple.
h
,
r
and
t
are called a triple’s head entity (subject), relation
(predicate) and tail entity (object), respectively. KGC is then
defined to predict an input candidate triple as true or not (i.e.,
triple classification), or predict the missing entity/relation in a
triple with the other two elements given (i.e., entity/relation
prediction), which is often achieved by filling the incomplete
triple with a candidate entity/relation and feeding it into the
model. It is expected that the true (positive) triples are predicted
with higher scores than those false (negative) ones [2]. In a
commonly practiced partially inductive KGC setting, a set of
unseen entities
E0
with
E ∩ E0=∅
are newly emerged during
prediction, the goal is then to predict a triple (
h, r, t
) with
r∈ R
and
h, t ∈ E0
.
G
is often known as the training graph,
while the graph composed of the triples by the unseen entities
E0and the relations Ris often known as the testing graph.
In this study, we extend the above partially inductive KGC
to fully inductive KGC by introducing a set of unseen relations
R0
with
R∩R0=∅
to the testing graph. Specially, we
further investigate two situations: one is a general testing graph
involving both seen and unseen relations, while the other is
a testing graph involving only unseen relations. Formally, a
triple given in the testing graph and a triple to predict can be
denoted as (
h, r, t
) with
h, t ∈ E0
,
r∈(R∪R0)
or
r∈ R0
.
We name the evaluation with the first testing graph as testing
with semi unseen relations, and name the evaluation with the
second testing graph as testing with fully unseen relations.
B. Subgraph-based Partially Inductive Reasoning
To predict a target triple (
h, r, t
) where
r∈ R
,
h, t ∈ E0
, and
the embeddings of
h
and
t
are both not available, methods such
as GraIL [31] learn and utilize the structural semantics from the
subgraph around this triple in an entity-independent manner. To
better understand, we re-denote such a triple as (
u, rt, v
) using
the notations widely used in GNN, where
u
and
v
are referred
as target head entity and target tail entity, respectively, and
rt
is the target relation. The basic workflow of GraIL includes
three steps. It first extracts the
K
-hop enclosing subgraph
2Resource Description Framework. See https://www.w3.org/RDF/.