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R8244/node_modules/mongoose/examples/population/population-across-three-collections.js
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population-across-three-collections.js
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			var assert = require('assert');
			var mongoose = require('../../lib');
			var Schema = mongoose.Schema;
			var ObjectId = mongoose.Types.ObjectId;

			/**
			* Connect to the db
			*/

			var dbname = 'testing_populateAdInfinitum_' + require('../../lib/utils').random();
			mongoose.connect('localhost', dbname);
			mongoose.connection.on('error', function() {
			console.error('connection error', arguments);
			});

			/**
			* Schemas
			*/

			var user = new Schema({
			name: String,
			friends: [{
			type: Schema.ObjectId,
			ref: 'User'
			}]
			});
			var User = mongoose.model('User', user);

			var blogpost = Schema({
			title: String,
			tags: [String],
			author: {
			type: Schema.ObjectId,
			ref: 'User'
			}
			});
			var BlogPost = mongoose.model('BlogPost', blogpost);

			/**
			* example
			*/

			mongoose.connection.on('open', function() {
			/**
			* Generate data
			*/

			var userIds = [new ObjectId, new ObjectId, new ObjectId, new ObjectId];
			var users = [];

			users.push({
			_id: userIds[0],
			name: 'mary',
			friends: [userIds[1], userIds[2], userIds[3]]
			});
			users.push({
			_id: userIds[1],
			name: 'bob',
			friends: [userIds[0], userIds[2], userIds[3]]
			});
			users.push({
			_id: userIds[2],
			name: 'joe',
			friends: [userIds[0], userIds[1], userIds[3]]
			});
			users.push({
			_id: userIds[3],
			name: 'sally',
			friends: [userIds[0], userIds[1], userIds[2]]
			});

			User.create(users, function(err) {
			assert.ifError(err);

			var blogposts = [];
			blogposts.push({
			title: 'blog 1',
			tags: ['fun', 'cool'],
			author: userIds[3]
			});
			blogposts.push({
			title: 'blog 2',
			tags: ['cool'],
			author: userIds[1]
			});
			blogposts.push({
			title: 'blog 3',
			tags: ['fun', 'odd'],
			author: userIds[2]
			});

			BlogPost.create(blogposts, function(err) {
			assert.ifError(err);

			/**
			* Population
			*/

			BlogPost
			.find({tags: 'fun'})
			.lean()
			.populate('author')
			.exec(function(err, docs) {
			assert.ifError(err);

			/**
			* Populate the populated documents
			*/

			var opts = {
			path: 'author.friends',
			select: 'name',
			options: {limit: 2}
			};

			BlogPost.populate(docs, opts, function(err, docs) {
			assert.ifError(err);
			console.log('populated');
			var s = require('util').inspect(docs, {depth: null, colors: true});
			console.log(s);
			done();
			});
			});
			});
			});
			});

			function done(err) {
			if (err) console.error(err.stack);
			mongoose.connection.db.dropDatabase(function() {
			mongoose.connection.close();
			});
			}