As title, for summary rows we are currently serializing the row with the incorrect subtable ID, causing it not to load w/ the subtable.
@tsteur from the blame it looks like we were never serializing the summary row. The specific line was last changed seven years ago: https://github.com/matomo-org/matomo/blame/b77e5ff594cdae59862dd4d863220baffc200bec/core/DataTable.php#L1323
And the summaryRow property has been there for nine years. So I think this has always been an issue.
Do you know how this later detects the summary row in methods like addRowsFromSerializedArray where it relied on having the summary row ID?
We serialize the rows array which associates id w/ row data, so we simply unserialize it. The subtable ID after directly unserializing is used to get the subtable in the database. So it detects it because of the ID we serialize it with.
I'm not sure if this answers your questions...
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