MongoDB - Only fetch the first item in an array on a populated document - e.g. user.populate('posts', 'images.0')

Question

Say I'm fetching a user with a posts field. Each post has a field images. I want to populate the posts on the user, but only fetch the first image from each post. How can I do this? I've given the following as an example that does not work:

db.User.findOne().populate('posts', 'images.0')

Answer 1

You can use $slice projection operator to specifies the number of elements in an array to return in the query result.

db.User.findOne().populate('posts', { 'images': { '$slice': 1 } })

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If you want to select direct string instead of array of string use $arrayElemAt or $first aggregation operator Starting from MongoDB 4.4,

• $arrayElemAt

db.User.findOne().populate('posts', { 'images': { '$arrayElemAt': ["$images", 0] } })

• $first

db.User.findOne().populate('posts', { 'images': { '$first': "$images" } })