Problem:
How to use MongoDb 3 API to filter, sort, and project data? Unlike previous versions of Mongo, the new version there provides convenient API to work with data.
In previous versions of MongoDB we had to create queries by putting together Document tree, which was very inconvenient. This approach still does work, but Mongo 3 shipped with bunch of flexible classes to make it a lot easier:
- com.mongodb.client.model.Filters
- com.mongodb.client.model.Projections
- com.mongodb.client.model.Sorts
Solution:
In this example we’re going to explore the following data:
{"name": "TYH", "score": 962}
{"name": "QGV", "score": 814}
{"name": "WOJ", "score": 686}
{"name": "QES", "score": 600}
{"name": "VJU", "score": 590}
{"name": "LHZ", "score": 581}
{"name": "RHS", "score": 453}
{"name": "FPG", "score": 214}
{"name": "LUW", "score": 155}
{"name": "WOJ", "score": 25}
To import that in to MongoDB, put the data into a file (let’s say highscores.json) and import using the following command:
mongoimport -d test -c highscores < highscores.json
Now we can explore the data. In the following code we’re going to show how to use MongoDB 3 API to create filters, sort data, and select document fields (projections):
package com.farenda.mongodb;
import com.mongodb.client.FindIterable;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.model.Filters;
import com.mongodb.client.model.Projections;
import com.mongodb.client.model.Sorts;
import org.bson.Document;
import org.bson.conversions.Bson;
import static java.lang.System.out;
public class FindProjectSortExample {
private MongoConnector connector = new MongoConnector();
private MongoCollection<Document> scores
= connector.collection("highscores");
public static void main(String[] args) {
FindProjectSortExample example = new FindProjectSortExample();
example.count();
example.sortingExample();
example.projectionExample();
example.filteringUsingLogicalOperator();
example.filteringUsingRegexp();
example.compoundFiltering();
}
private void count() {
out.println("Total number of scores: "
+ scores.count());
}
private void sortingExample() {
out.println("\nBest 3 by score:");
FindIterable<Document> best3 = scores
.find()
.limit(3)
.sort(Sorts.descending("score"));
printResults(best3);
}
private void projectionExample() {
out.println("\nThe best score:");
FindIterable<Document> best = scores
.find()
.limit(1)
.sort(Sorts.descending("score"))
// "name" will be null in output
.projection(Projections.include("score"));
printResults(best);
}
private void filteringUsingLogicalOperator() {
out.println("\nTake first where name is 'WOJ' or 'RHS':");
Document doc = scores
.find(Filters.or(
Filters.eq("name", "WOJ"),
Filters.eq("name", "RHS")))
// Take only the first one:
.first();
if (doc != null) {
out.printf("Name: %s, score: %d%n",
// Note the methods to remove casting:
doc.getString("name"),
doc.getInteger("score"));
}
}
private void filteringUsingRegexp() {
Bson nameRegexp = Filters.regex("name", "L");
out.printf("\nScores matching 'L' regexp (%d):%n",
scores.count(nameRegexp));
printResults(scores.find(nameRegexp));
}
private void compoundFiltering() {
out.println("\nTake scores >= 500, skip 2, limit to 5:");
FindIterable<Document> results = scores
.find()
// Skip is third!
.skip(2)
// Limit is at the end!
.limit(5)
// Filtering goes second:
.filter(Filters.gte("score", 500))
// Sorting is done first!
.sort(Sorts.orderBy(
Sorts.descending("score"),
Sorts.ascending("name")));
printResults(results);
}
private void printResults(FindIterable<Document> results) {
for (Document doc : results) {
out.printf("Name: %s, score: %d%n",
doc.getString("name"),
doc.getInteger("score"));
}
}
}
In the code above we use MongoConnector, which can be easily constructed as in the previous post.
And here’s the output of running the above program:
Total number of scores: 10 Best 3 by score: Name: TYH, score: 962 Name: QGV, score: 814 Name: WOJ, score: 686 The best score: Name: null, score: 962 Take first where name is 'WOJ' or 'RHS': Name: RHS, score: 453 Scores matching 'L' regexp (2): Name: LHZ, score: 581 Name: LUW, score: 155 Take scores >= 500, skip 2, limit to 5: Name: WOJ, score: 686 Name: QES, score: 600 Name: VJU, score: 590 Name: LHZ, score: 581
There are a few things to note:
- The name in Best score in null, because in Projection we specified that we want only score field.
- In logical operator example we display only one value, because we took only the first one from the result set.
- The last example shows the order of operations: sort, filter, skip, and limit (from 6 scores above 500, 2 were skipped, so limit did nothing, because only 4 have left).
