BigQuery UPDATE of column within nest from GoogleAnalytics export - google-bigquery

Question

I have a website's Google Analytics tracking data exported in BigQuery to a ga_session_* table. It's the standard setup of any GA export to BQ I've come across.
For simplicity's sake I'll refer to only ga_session_20220801.
This table has the following fields:
- visitorId INTEGER NULLABLE
- visitNumber INTEGER NULLABLE
- visitId INTEGER NULLABLE
- visitStartTime INTEGER NULLABLE
- date STRING NULLABLE
- totals RECORD NULLABLE
- trafficSource RECORD NULLABLE
- device RECORD NULLABLE
- geoNetwork RECORD NULLABLE
- customDimensions RECORD REPEATED
- hits RECORD REPEATED
- fullVisitorId STRING NULLABLE
- userId STRING NULLABLE
- clientId STRING NULLABLE
- channelGrouping STRING NULLABLE
- socialEngagementType STRING NULLABLE
- privacyInfo RECORD NULLABLE
Field hits is a repeated field i.e. it contains multiple records for each table record.
My question
How can I execute an update statements on field hits.sourcePropertyInfo.sourcePropertyTrackingId, where sourcePropertyInfo is another nest inside hits (but not a repeated record)?
My attempts
update `my_project.my_dataset.ga_sessions_20220801`
set hits.sourcePropertyInfo.sourcePropertyTrackingId = 'some value'
where 1=1
Cannot access field sourcePropertyInfo on a value with type ARRAY<STRUCT<hitNumber INT64, time INT64, hour INT64, ...>> at [2:10]
update `my_project.my_dataset.ga_sessions_20220801`
set (select sourcePropertyInfo.sourcePropertyDisplayName from unnest(hits)) = 'some value'
where 1=1
Syntax error: Expected keyword DELETE or keyword INSERT or keyword UPDATE but got keyword SELECT at [2:6]
The following is my only attempt that was not stopped by syntax. I ended up trying to re-create the whole nest by listing each field. But still returns an error at runtime due to the record being repeated.
update `my_project.my_dataset.ga_sessions_20220801`
set hits = ARRAY(
SELECT AS STRUCT
hits.hitNumber, hits.time, hits.hour, hits.minute, hits.isSecure, hits.isInteraction, hits.isEntrance, hits.isExit, hits.referer, hits.page,
(SELECT AS STRUCT hits.transaction.transactionId, hits.transaction.transactionRevenue, hits.transaction.transactionTax, hits.transaction.transactionShipping, hits.transaction.affiliation,
hits.transaction.currencyCode,
hits.transaction.localTransactionRevenue, hits.transaction.localTransactionTax, hits.transaction.localTransactionShipping, hits.transaction.transactionCoupon)
as transaction,
hits.item, hits.contentInfo, hits.appInfo, hits.exceptionInfo, hits.eventInfo, hits.product, hits.promotion, hits.promotionActionInfo, hits.refund, hits.eCommerceAction, hits.experiment, hits.publisher, hits.customVariables, hits.customDimensions, hits.customMetrics, hits.type, hits.social, hits.latencyTracking,
(SELECT AS STRUCT 'some value' as sourcePropertyDisplayName, hits.sourcePropertyInfo.sourcePropertyTrackingId)
as sourcePropertyInfo,
hits.contentGroup, hits.dataSource, hits.publisher_infos, hits.uses_transient_token
FROM gas.hits
)
where 1=1
Scalar subquery produced more than one element

Answer 1

You're so close with the third query. You should recreate the hits column, but by preserving the original data structure.
In the query below, I get all the hit rows and replace their sourcePropertyInfo key in the struct.
Then, since I unnested the hits, to gather it again, I used array_agg so it becomes an array again.
update `my_project.my_dataset.ga_sessions_20220801`
set hits =
(
select array_agg(t)
from (
select
hit.* replace(
struct(
hit.sourcePropertyInfo.sourcePropertyDisplayName,
'some value' as sourcePropertyTrackingId
) as sourcePropertyInfo
)
from unnest(hits) as hit
) as t
)
where true