ensureIndex
ensureIndex is a method primarily associated with older versions of MongoDB, specifically prior to version 3.0. Its purpose was to create an index on a collection if one did not already exist. If an index with the same key specification already existed, ensureIndex would do nothing. If an index with conflicting specifications existed, it would raise an error. This method was useful for ensuring that specific fields were indexed, which is crucial for efficient query performance.
The syntax for ensureIndex typically involved specifying the fields to be indexed and optionally the index
In MongoDB 3.0 and later, ensureIndex was deprecated and replaced by the more general createIndex method. The