How to Use MongoDB's $in Operator with QueryBuilder in Java to Retrieve Objects

Why does the $in operator with QueryBuilder in MongoDB Java return no results?

// Example of using QueryBuilder with $in
List<String> values = Arrays.asList("val1", "val2", "val3");
Bson filter = Filters.in("fieldName", values);
FindIterable<Document> result = collection.find(filter);

Using the $in operator with QueryBuilder in MongoDB allows you to filter documents based on whether a field's value matches any value in a specified list. However, returning no results can occur for various reasons. This guide explains how to ensure correct implementation and troubleshoot common issues.

// Correct usage of $in with QueryBuilder
List<String> values = Arrays.asList("val1", "val2"); // Ensure this matches the document field type
Bson filter = Filters.in("fieldName", values);
FindIterable<Document> result = collection.find(filter);

Causes

• The field name specified in the $in operator does not match the actual field name in the collection.
• The data types of the values in the list do not match the data types of the corresponding field values in the documents.
• The list provided to the $in operator is empty, leading to no matching records.
• Other filters are applied to the query that conflict with the $in condition, resulting in an empty result set.

Solutions

• Double-check the field name used in the $in operator and ensure it matches exactly (case-sensitive) with the field name in your MongoDB documents.
• Verify that the data types of the values in your list match the data types of the document fields they're being compared against.
• Make sure that the list you're passing to the $in operator is not empty; it should contain at least one value.
• Review any additional filters applied alongside the $in operator to ensure they don't unintentionally exclude matches.

Mistake: Using an incorrect field name in the $in operator.

Solution: Always verify that the field name is correct and is case-sensitive.

Mistake: Providing values of different types than what's stored in MongoDB.

Solution: Check the data types in your MongoDB collection and ensure they match the types of the values you're querying.

Mistake: Having an empty list input for the $in operator.

Solution: Ensure that your list contains valid entries for a successful query.

Mistake: Overly restrictive additional filters leading to no results.

Solution: Carefully review combined filters to understand their interaction and their potential impact on the results.