Reshaping Documents in $project Stages - node.js

Question

For our collection which looks like this:
> db.companies.find().limit(1).pretty() {
"_id": ObjectId("52cdef7c4bab8bd675297d8b"),
"name": "AdventNet",
"permalink": "abc3",
"crunchbase_url": "http://www.crunchbase.com/company/adventnet",
"homepage_url": "http://adventnet.com",
"blog_url": "",
"blog_feed_url": "",
"twitter_username": "manageengine",
"category_code": "enterprise",
"number_of_employees": 600,
"founded_year": 1996,
"deadpooled_year": 2,
"tag_list": "",
"alias_list": "Zoho ManageEngine ",
"email_address": "pr#adventnet.com",
"phone_number": "925-924-9500",
"description": "Server Management Software",
"created_at": ISODate("2007-05-25T19:24:22Z"),
"updated_at": "Wed Oct 31 18:26:09 UTC 2012",
"overview": "<p>AdventNet is now <a href=\"/company/zoho-manageengine\"
title=\"Zoho ManageEngine\" rel=\"nofollow\">Zoho ManageEngine</a>.</p>\n\n<p>F
ounded in 1996, AdventNet has served a diverse range of enterprise IT, networkin
g and telecom customers.</p>\n\n<p>AdventNet supplies server and network managem
ent software.</p>",
"image": {
"available_sizes": [
[
[
150,
55
],
"assets/images/resized/0001/9732/19732v1-max-150
x150.png"
],
[
[
150,
55
],
"assets/images/resized/0001/9732/19732v1-max-250
x250.png"
],
[
[
150,
55
],
"assets/images/resized/0001/9732/19732v1-max-450
x450.png"
]
]
},
"products": [],
"relationships": [{
"is_past": true,
"title": "CEO and Co-Founder",
"person": {
"first_name": "Sridhar",
"last_name": "Vembu",
"permalink": "sridhar-vembu"
}
}, {
"is_past": true,
"title": "VP of Business Dev",
"person": {
"first_name": "Neil",
"last_name": "Butani",
"permalink": "neil-butani"
}
}, {
"is_past": true,
"title": "Usabiliy Engineer",
"person": {
"first_name": "Bharath",
"last_name": "Balasubramanian",
"permalink": "bharath-balasibramanian"
}
}, {
"is_past": true,
"title": "Director of Engineering",
"person": {
"first_name": "Rajendran",
"last_name": "Dandapani",
"permalink": "rajendran-dandapani"
}
}, {
"is_past": true,
"title": "Market Analyst",
"person": {
"first_name": "Aravind",
"last_name": "Natarajan",
"permalink": "aravind-natarajan"
}
}, {
"is_past": true,
"title": "Director of Product Management",
"person": {
"first_name": "Hyther",
"last_name": "Nizam",
"permalink": "hyther-nizam"
}
}, {
"is_past": true,
"title": "Western Regional OEM Sales Manager",
"person": {
"first_name": "Ian",
"last_name": "Wenig",
"permalink": "ian-wenig"
}
}],
"competitions": [],
"providerships": [{
"title": "DHFH",
"is_past": true,
"provider": {
"name": "A Small Orange",
"permalink": "a-small-orange"
}
}],
"total_money_raised": "$0",
"funding_rounds": [],
"investments": [],
"acquisition": null,
"acquisitions": [],
"offices": [{
"description": "Headquarters",
"address1": "4900 Hopyard Rd.",
"address2": "Suite 310",
"zip_code": "94588",
"city": "Pleasanton",
"state_code": "CA",
"country_code": "USA",
"latitude": 37.692934,
"longitude": -121.904945
}],
"milestones": [],
"video_embeds": [],
"screenshots": [{
"available_sizes": [
[
[
150,
94
],
"assets/images/resized/0004/3400/43400v1
-max-150x150.png"
],
[
[
250,
156
],
"assets/images/resized/0004/3400/43400v1
-max-250x250.png"
],
[
[
450,
282
],
"assets/images/resized/0004/3400/43400v1
-max-450x450.png"
]
],
"attribution": null
}],
"external_links": [],
"partners": []
} >
For the below query for our node.js app.
db.companies.aggregate([{
$match: {
"name": "Facebook"
},
$project: {
"_id": 0,
"name": 1,
"people": "$relationships.person.last_name"
}
}])
Gives the following error:
assert: command failed: {
"ok" : 0,
"errmsg" : "A pipeline stage specification object must contain exactly o ne field.",
"code" : 16435 } : aggregate failed
_getErrorWithCode#src/mongo/shell/utils.js:25:13 doassert#src/mongo/shell/assert.js:13:14
assert.commandWorked#src/mongo/shell/assert.js:267:5
DBCollection.prototype.aggregate#src/mongo/shell/collection.js:1312:5
#(shell):1:1
2016-09-17T19:20:26.303+0530 E QUERY [thread1] Error: command
failed: {
"ok" : 0,
"errmsg" : "A pipeline stage specification object must contain exactly o ne field.",
"code" : 16435 } : aggregate failed :
_getErrorWithCode#src/mongo/shell/utils.js:25:13 doassert#src/mongo/shell/assert.js:13:14
assert.commandWorked#src/mongo/shell/assert.js:267:5
DBCollection.prototype.aggregate#src/mongo/shell/collection.js:1312:5
#(shell):1:1
I'm unable to figure out why?

Answer 1

You are missing some braces for your aggregation pipeline operators. The correct pipeline should be:
db.companies.aggregate([
{ "$match": { "name": "Facebook" } }, /* match pipeline stage */
{ "$project": { /* project pipeline stage */
"_id": 0,
"name": 1,
"people": "$relationships.person.last_name"
} }
])