Behavior differences

This page describes behavioral differences between Firestore with MongoDB compatibility and MongoDB.

For a breakdown of supported features depending on MongoDB version, see:

Connections and databases

  • Each connection is limited to a single Firestore with MongoDB compatibility database.
  • A database must be created before connecting to it.

Naming

The following differences apply to naming parts of your data model.

Collections

  • Collection names matching __.*__ are not supported.

Fields

  • Field names matching __.*__ are not supported.
  • Empty field names are not supported.

Documents

  • The maximum document size is 4 MiB.
  • The maximum nesting depth of fields is 20. Each Array and Object-typed field adds one level to the overall depth.

_id

  • Document _id (top-level field) must be an ObjectId, String, or 64-bit integer. Other BSON types are not supported.
  • Empty string ("") and 64-bit 0 (0L) are not supported.

Values

  • The JavaScript, Symbol, DBPointer, and Undefined BSON types are not supported.

Date

  • Date values must fall in [0001-01-01T00:00:00Z, 9999-12-31T23:59:59Z].

Decimal128

  • NaN, positive infinity, and negative infinity values are canonicalized on write.
  • Arithmetic operations on Decimal128 are not supported.

Double

  • NaN values are canonicalized on write.

Regular expression

  • Regular expression options must be valid ("i", "m", "s", "u", or "x") and provided in alphabetical order without repeats.

Queries

  • Natural sort order (queries without an explicit sort) does not match insertion order or order by _id ascending.

Aggregations

  • Aggregations are limited to 250 stages.
  • The $merge and $out stages are not supported. See the commands section for a complete list of supported stages and operators.

Writes

  • Documents with names beginning with a dollar sign ("$") cannot be created using the upsert feature of update or findAndModify.
  • Make sure your connection string includes retryWrites=false (or use the method appropriate to your driver) to ensure the driver does not attempt to use this feature. Retryable writes are not supported.

Transactions

  • Snapshot transactions are not supported.
  • Transactions use pessimistic concurrency controls with locking. To avoid lock contention, limit the amount of data read in a transaction.

Indexes

  • Wildcard indexes are not supported.
  • Unique indexes are not supported
  • Firestore with MongoDB compatibility does not automatically create an index on _id, but it ensures values of _id are unique within a collection.
  • Indexes without multi-key enabled are not automatically changed to multi-key indexes based on write operations. You must enable multi-key when you create the index and the option cannot be changed.

Read concern

  • Only the majority read concern level is supported.

Write concern

  • Only w: 'majority' and w: 1 write concerns are supported.

Read preference

  • Only the primary, primaryPreferred, primary_preferred, secondary_preferred, and nearest read concerns are supported.

Errors

  • Error codes and messages may differ between Firestore with MongoDB compatibility and MongoDB.

Commands

The following behavior differences apply to specific commands.

  • Commands not listed in the following tables are unsupported.
  • maxTimeMS is accepted by most commands but may be ignored.

Queries and writes

Command Unsupported Fields

find

  • comment
  • readConcern
  • max
  • min
  • returnKey
  • showRecordId
  • tailable
  • oplogReplay
  • noCursorTimeout
  • awaitData
  • allowPartialResults
  • collation
  • allowDiskUsage
  • let

aggregate

  • bypassDocumentValidation
  • readConcern
  • collation
  • hint
  • comment
  • let

insert

  • bypassDocumentValidation
  • comment

update

  • collation
  • arrayFilters
  • hint

delete

  • comment
  • write

Within a delete statement:

  • collation
  • hint

findAndModify

  • fields
  • bypassDocumentValidation
  • collation
  • arrayFilters
  • hint
  • comment
  • let

count

  • hint
  • readConcern
  • collation
  • comment

distinct

  • readConcern
  • collation
  • comment
  • hint

getMore

  • comment

killCursors

(none)

Transactions and sessions

Command Unsupported Fields

commitTransaction

  • comment

abortTransaction

  • comment

endSessions

(none)

Administration

Command Unsupported Fields Notes

listDatabases

  • authorizedDatabases
  • comment
 filter must be empty if provided.

listCollections

  • comment
 authorizedCollections must be false if provided.

listIndexes

  • comment

createCollection

  • timeseries
  • expireAfterSeconds
  • clusteredIndex
  • changeStreamPreAndPostImages
  • size
  • max
  • storageEngine
  • validator
  • validationLevel
  • validationAction
  • indexOptionDefaults
  • viewOn
  • pipeline
  • collation
  • writeConcern
  • encryptedFields
  • comment
 This command is a no-op.

capped must be false if provided.

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