Restrict an array size up to N indices (Security Rules)

Question

Consider the following maximum array size:

Now let's say a user performs a push() childByAutoId. What I expect to happen is

if (arraySize > arraySizeLimit) {
// Delete first Index
}

I am fairly new to firebase security rules. I did some reading but still struggling with this. I apologise for not having any "attempt" code to present.

Answer 1

There's no way for security rules to check the size of a list like that.

Th main options that I know of, are:

• Kato's age-old answer to this question, which uses known node names to impose a limit: Limit number of records that can be written to a path (reference other paths in security rules)
• This extension or Cloud Function that reduces the number of child nodes to the maximum configured after a write operation goes beyond that number.
• Perform the write through a single Cloud Function, which the app calls. In the Cloud Function you'd then read the entire messages node and truncate it as needed.