Best Practices: Arrays in Firebase

Firebase does not natively store arrays, instead representing them as objects with numeric keys and rendering them as arrays only when the keys are sufficiently sequential, which can change after deletions. Because array indices are mutable, concurrent clients can unintentionally modify or delete the wrong records, making arrays unreliable for distributed, collaborative data. The recommended approach is to store collections as objects keyed by permanent unique identifiers generated with Firebase’s push method, while using priorities to control retrieval order for uses such as leaderboards. Arrays may still be appropriate when only one client writes at a time, the complete array is saved after changes, and applications avoid relying on mutable indices; Java and iOS clients can also cast array-like Firebase data into platform-specific list or array types.

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