A vest in one table pair instance_id, instance_type in Doctrine

Use Doctrine ODM + MongoDB :)
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If you do not fit this option, consider creating a common PrimaryKey (maybe some more fields will be common) for instance, that is, instead of instance_id and instance_type log is stored inctance_id, but it is common for the two tables and the linkage occurs either directly through him (then you have to ensure the uniqueness of id in both tables, a simple auto-increment will not work), or you can create another table with fields id (auto-increment, in which communication goes from the log), instance1_id, instance2_id + other common fields. In General, the pattern of "inheritance tables for each class" (if not confused).
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Once tried to solve a similar problem, but I have a number of types were unlimited and one SQL was not able to solve it

Why not add a link as the Doctrine?
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participant hasMany log
team hasMany log
region hasMany log
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communication on the field (instance_id -> id)
And in the query add the condition for the instance_type and target model.