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doc/development/object_state_models.md

 # Object state models
 ## Contents
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 * [Diagrams](#diagrams)
 * [Legend](#legend)
 * [Issue](#issue)
 * [Merge request](#merge-request)
 ## Diagrams
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 [GitLab object state models](https://drive.google.com/drive/u/3/folders/0B5tDlHAM4iZINmpvYlJXcDVqMGc)
 ---
 ## Legend
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 ![legend](img/state-model-legend.png)
 --- 
 ## Issue
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 [`app/models/issue.rb`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/issue.rb)
 ![issue](img/state-model-issue.png)
 --- 
 ## Merge request
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 [`app/models/merge_request.rb`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/merge_request.rb)
 ![merge request](img/state-model-merge-request.png)
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