You perform the following operation in the shell: db.foo.insert( { } ); What gets inserted?

Question

Options

• AA document will be inserted with the same _id as the last document inserted
• BA document that matches the collection's existing schema, but with null fields
• CA document with an _id assigned to be an Objectld
• DAn empty document
• ENo document will be inserted; an error will be raised

• A
  5% (3)
• B
  3% (2)
• C
  90% (57)
• D
  2% (1)

Explanation

When you insert {} into MongoDB, the database automatically generates and appends an _id field with a unique ObjectId value before writing the document - making C correct. MongoDB requires every document to have an _id, so if you don't supply one, it creates it for you.

Why the distractors fail:

• A - MongoDB never reuses or copies a previous document's _id; each ObjectId is globally unique by design.
• B - MongoDB is schemaless; collections have no enforced schema to match, and there is no concept of "null fields" derived from other documents.
• D - Tempting, but not quite right: the stored document is { _id: ObjectId("...") }, not truly empty.
• E - MongoDB accepts empty documents without complaint; no error is raised.

Memory tip: Think of MongoDB as a "stamp machine" - every document that enters the collection gets an _id stamp automatically if you don't bring your own, so an empty {} always comes out as { _id: ObjectId(...) }.

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