FIX: Stop silently dropping first two rows during load_mapping
#33076
+17
−5
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Currently, the first two rows returned by `DiscourseDB#query_array` are silently dropped during the column size check in `DiscourseDB#load_mapping`. This happens because the rows object, while an enumerator, isn't fully compliant, it doesn't rewind during introspection. As a result, calls like `#first`, `#peek`, or `#any?` advance the iterator. Ideally, we’d fix this by updating the `query_array` enumeration implementation. However, customizing the enumerator to be fully compliant would likely introduce unnecessary perf overhead for all use cases. So, this fix works around that limitation by building the map a little differently.
gschlager
approved these changes
Jun 9, 2025
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Good catch!
I guess we should make it more explicit by using a custom enumerator in DiscourseDB to avoid making this mistake in the future.
class NonRewindableEnumerator < Enumerator
def rewind
raise RuntimeError, "Rewind not supported on this enumerator"
end
end|
Thanks! I explored extending behaviour with a custom enumerator but decided it'd add unneeded overhead at this point. I think having a custom enumerator which simply throws an exception might be simpler |
martin-brennan
pushed a commit
that referenced
this pull request
Jun 10, 2025
) Currently, the first two rows returned by `DiscourseDB#query_array` are silently dropped during the column size check in `DiscourseDB#load_mapping`. This happens because the rows object, while an enumerator, isn't fully compliant, it doesn't rewind during introspection. As a result, calls like `#first`, `#peek`, or `#any?` advance the iterator. Ideally, we’d fix this by updating the `query_array` enumeration implementation. However, customizing the enumerator to be fully compliant would likely introduce unnecessary perf overhead for all use cases. So, this fix works around that limitation by building the map a little differently.
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Currently, the first two rows returned by
DiscourseDB#query_arrayare silently dropped during the column size check inDiscourseDB#load_mapping. This happens because the rows object, while an enumerator, isn't fully compliant, it doesn't rewind during introspection. As a result, calls like#first,#peek, or#any?advance the iterator.Ideally, we’d fix this by updating the
query_arrayenumeration implementation. However, customizing the enumerator to be fully compliant would likely introduce unnecessary perf overhead for all use cases. So, this fix works around that limitation by building the map a little differently.