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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Foreign key to a union","articleBody":"In many applications, a need arises to converge two pipelines into one.  A desired feature would be to have a foreign key into the union of two tables.  For example, imagine that two schemas `s1` and `s2` have the table `Scan`.   Both `s1.Scan` and `s2.Scan` have the same primary key structure but the tables contain non-overlapping sets of primary key values.  \r\n\r\nSuppose we need to compute some statistic of the scan in the table `ScanStat` for the data in both `s1.Scan` and `s2.Scan`. \r\n\r\nWe want to be able to declare\r\n\r\n```python\r\n@schema\r\nclass ScanStat(dj.Computed):\r\n    definition = \"\"\"  # Scan statistic\r\n    -\u003e s1.Scan + s2.Scan\r\n    ----\r\n    scan_stat  : double\r\n    \"\"\"\r\n```","author":{"url":"https://github.com/dimitri-yatsenko","@type":"Person","name":"dimitri-yatsenko"},"datePublished":"2017-06-14T17:53:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/324/datajoint-python/issues/324"}