Docs Menu

Compatibility Changes with Legacy mongo Shell

On this page

  • Deprecated Methods
  • Read Preference Behavior
  • Numeric Values

The following behaviors in mongosh affect compatibility with operations performed in the legacy mongo shell.

The following shell methods are deprecated in mongosh. Instead, use the methods listed in the Alternative Resources column.

Deprecated Method
Alternative Resources
Aggregation stage: $out
No longer required. See Read Operations on a Secondary Node.

When using mongosh to connect directly to a secondary replica set member, you do not have to explicitly enable secondary reads with rs.secondaryOk().

You can read from a secondary member if you specify a read preference of either:

To specify a read preference, you can use either:

The legacy mongo shell required running rs.secondaryOk() to enable secondary reads.

The following show helper methods always use a read preference of primaryPreferred, even when a different read preference has been specified for the operation:

  • show dbs
  • show databases
  • show collections
  • show tables

In the legacy mongo shell, these operations use the specified read preference.

The legacy mongo shell stored numerical values as doubles by default. In mongosh numbers are stored as 32 bit integers, Int32, or else as Double if the value cannot be stored as an Int32.

MongoDB Shell continues to support the numeric types that are supported in mongo shell. However, the preferred types have been updated to better align with the MongoDB drivers. See mongosh Data Types for more information.

The preferred types for numeric variables are different in MongoDB Shell than the types suggested in the legacy mongo shell. The types in mongosh better align with the types used by the MongoDB Drivers.

mongo type
mongosh type

Data types may be stored inconsistently if you connect to the same collection using both mongosh and the legacy mongo shell.

See also:

For more information on managing types, refer to the schema validation overview.

Give Feedback
© 2021 MongoDB, Inc.


  • Careers
  • Legal Notices
  • Privacy Notices
  • Security Information
  • Trust Center
© 2021 MongoDB, Inc.