###### tags: Paper Reading
# Matching the Blanks: Distributional Similarity for Relation Learning
## Outline
    This paper is talking about building and training a general purpose relation extractors with Bert.
## Reference
    ACL 2019
## Introduction/Motivation
    Building a relation extractor between any two entities is a long standing goal. Previous work try to use **surface forms**,**joint representation surface forms**, and learing **word embedding** to represent entities' relations. All works above have common result that the extractor is not general enough for different task. So, this paper take use of Bert and concentrate to learning mappings from relation statements to relation representations(Sectoin 2, task definition)

## Model
   Because Bert has not been used in relation represent problem, it propose some variant of inputs and outputs of Bert encoder.
*     **input**
* Standard input
      Do not specify position of entity
* Positional embeddings
      Use segmentation embedding(one of Bert's input). Put all tokens segement types of entity_1 into 1 (entity_2 into 2)
* Entity marker tokens
      Add reserved word into front and end of entities.(E1_start,E1_end,E2_start,E2_end)
*     **output**
* [CLS] token
      Adopt the **[CLS]** output, as relation representation.
* Entity mention pooling
      **Maxpool** all tokens' final hidden layer output respect two entities. And concat them.
* Entity start state
       Use concatenation of **output from E1_start and E2_start** as final representation
## Matching the blanks
   Based on the above assumptions, if **two sentences have same two entities, the representation of two sentences should as similar as possible.** As we know information of entities , so we can minimise loss after checking whether entities between two sentences are the same.
Therefore, this paper randomly(p=0.7) replace entity into reserved word **[blank]** to build relation extractor pretrained model.
## Experiment
    section5
## Conclusion
    This paper propose a noval traing setup: matching the blanks, give us a new idea to generalize our model. It's also propose new architecture for us to use Bert into relation extraction task. Excellent work.