Text indexes can apply to specific fields, or the entire document:

db.c.createIndex({ a: 'text' })

db.c.createIndex({ '$**': 'text' })

The second case is called a wildcard text index, not to be confused with wildcard indexes.

Wildcard indexes can apply to the whole document, or a subdocument:

db.c.createIndex({ '$**': 1 })

db.c.createIndex({ 'a.$**': 1 })

To complete the analogy, we should support creating a text index on an entire subdocument:

db.c.createIndex({ 'a.$**': 'text' })