What is the output of following two commands in MongoDB: db. posts. insert({n_id":l}) db.posts.insert({"_id":l})

Question

Options

• ATwo documents will be inserted with _id as 1
• BMongoDB will automatically increment the _id of the second document as 2
• CThis will throw a duplicate key error
• DIt will insert two documents and throw a warning to the user

• A
  4% (1)
• B
  4% (1)
• C
  93% (25)

Explanation

Option C is correct because MongoDB enforces a unique index on the _id field by default. Both commands attempt to insert a document with _id: 1, so the second insert throws E11000 duplicate key error - MongoDB rejects it entirely, no partial success.

Why the distractors are wrong:

• A is wrong because _id uniqueness is non-negotiable; MongoDB will never store two documents sharing the same _id.
• B is wrong because MongoDB only auto-generates an _id (as an ObjectId) when you omit it - it never silently increments an explicitly provided value.
• D is wrong because MongoDB does not have a "warn and continue" mode for unique index violations; it raises a hard error and the second document is not written.

Memory tip: Treat _id exactly like a SQL primary key - duplicate primary keys always throw errors, never warnings or auto-fixes.

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