Compare two arrays for intersection in GROQ query

For checking if two arrays have at least one intersection (overlapping value), you can use the count() function combined with the @ filter operator in GROQ. Here's the pattern:

count(array1[@ in array2]) > 0

So if you have a field allCheckinIds and a parameter $userIds, you'd write:

count(allCheckinIds[@ in $userIds]) > 0

This works by:

1. Taking the first array (allCheckinIds)
2. Filtering it with [@ in $userIds] - the @ represents each element in the array, and checks if it exists in the second array
3. Counting how many matches were found
4. Checking if that count is greater than 0

You can use this in a filter to find documents that have at least one matching value:

*[_type == "yourType" && count(allCheckinIds[@ in $userIds]) > 0]

This is a clever workaround since GROQ doesn't have a built-in array intersection operator. The technique comes from this community discussion where the same question was asked.

If you need to pass userIds as a parameter (which is common when querying from your application), you'd use it like this with the JS client:

client.fetch(
  `*[_type == "yourType" && count(allCheckinIds[@ in $userIds]) > 0]`,
  { userIds: ['id1', 'id2', 'id3'] }
)