Arrays haven't always been the best data structure for multi-user environments like Cloud Firestore. As Kato describes in the "Arrays are evil" section of his blog post, bad things can happen if you have multiple clients all trying to update or delete array elements at specific indexes. In the past, Cloud Firestore addressed these issues by limiting what you can do with arrays. That means that until now, you could really only "update" arrays by replacing the entire array (no appending or deleting!), and you couldn't perform meaningful queries on arrays, either.

This was problematic for those of you who wanted to use arrays in simple cases like keeping a list of tags or keywords. Well, with our latest improvements to arrays, none of this is necessary! For starters, we've added the ability to query for elements within arrays using the new "array-contains" feature. This means you can keep your elements as an array, and easily query for them without having to resort to the map hack.

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