Prevent spurious SQL query execution via ActiveSupport::Cache events #188
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When using a simple Rails view template like ```erb <% cache User.all do %> some content here <% end %> ``` we noticed that a spurious SQL query ```sql SELECT "users".* FROM "users" ``` was executed both on cache miss and on cache hit. Since the very point of [fragment caching][1] is to prevent this kind of queries, we investigated and found out that they were caused by `meta_request`. Rather than just stating the cache key used, events from `ActiveSupport::Cache` provide all original objects which contributed to the calculation of the cache key. This included `User.all` from the template. When `meta_request` checks whether this object is encodable to JSON ```ruby User.all.as_json(methods: [:duration]) ``` unintentionally triggers the spurious SQL query. We fix this by sanitizing the event payload and replacing the inputs of the cache key calculation with their final result. [1]: https://guides.rubyonrails.org/caching_with_rails.html#fragment-caching
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When using a simple Rails view template like
we noticed that a spurious SQL query
was executed both on cache miss and on cache hit.
Since the very point of fragment caching is to prevent this kind of queries, we investigated and found out that they were caused by
meta_request.Rather than just stating the cache key used, events from
ActiveSupport::Cacheprovide all original objects which contributed to the calculation of the cache key. This includedUser.allfrom the template.When
meta_requestchecks whether this object is encodable to JSONunintentionally triggers the spurious SQL query.
We fix this by sanitizing the event payload and replacing the inputs of the cache key calculation with their final result.