Nested queries for nested objects in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a nested query filtering by nested field

Given the following Elasticsearch nested query, what is the expected output when searching an index with documents containing nested 'comments' objects?

{
  "query": {
    "nested": {
      "path": "comments",
      "query": {
        "bool": {
          "must": [
            { "match": { "comments.author": "alice" } },
            { "range": { "comments.likes": { "gte": 10 } } }
          ]
        }
      }
    }
  }
}

Assuming the index has documents with nested comments by various authors with different likes, which option best describes the result?

A Documents that have comments with author 'alice' and comments with likes >= 10, but not necessarily in the same comment.

B Documents that have at least one comment by 'alice' with 10 or more likes.

C Documents that have all comments by 'alice' with 10 or more likes.

D Documents that have any comment by 'alice' or any comment with 10 or more likes.

💡 Hint

Remember that nested queries match nested objects individually, not across different nested objects.

The nested query filters documents where at least one nested 'comments' object matches both conditions: author is 'alice' and likes are 10 or more. It does not combine conditions across different nested objects.

🧠 Conceptual

intermediate

Understanding the purpose of nested queries

Why do we use nested queries in Elasticsearch when dealing with nested objects instead of regular queries?

A Because nested queries allow matching conditions on the same nested object, preserving object relationships.

B Because nested queries improve performance by indexing nested objects separately.

C Because nested queries automatically flatten nested objects into the root document.

D Because nested queries are required to search any field in Elasticsearch.

💡 Hint

Think about how nested objects are stored and how queries match them.

Nested queries ensure that conditions apply to the same nested object instance, preserving the relationship between fields inside that object. Regular queries might match fields from different nested objects incorrectly.

🔧 Debug

advanced

Identify the error in this nested query

What error will this Elasticsearch query produce?

{
  "query": {
    "nested": {
      "path": "comments",
      "query": {
        "match": {
          "author": "bob"
        }
      }
    }
  }
}

Assuming 'comments' is a nested field, what is wrong?

A It will cause a type mismatch error because 'author' is not a string field.

B It will run correctly and return documents with comments by 'bob'.

C It will cause a syntax error due to missing closing braces.

D It will cause a 'field [author] not found' error because the field path is incomplete inside nested query.

💡 Hint

Check the field name used inside the nested query's match clause.

Inside a nested query, the field names must be fully qualified with the nested path prefix. Using just 'author' instead of 'comments.author' causes Elasticsearch to not find the field.

📝 Syntax

advanced

Correct syntax for nested query with inner hits

Which of the following nested queries correctly includes inner hits to return matching nested objects?

A { "nested": { "path": "comments", "query": { "match": { "comments.text": "great" } }, "inner_hits": {} } }

B { "nested": { "path": "comments", "inner_hits": {}, "query": { "match": { "comments.text": "great" } } } }

C { "nested": { "query": { "match": { "comments.text": "great" } }, "path": "comments" } }

D { "nested": { "path": "comments", "query": { "match": { "text": "great" } }, "inner_hits": {} } }

💡 Hint

Check the order and presence of required keys and correct field names.

Option A correctly places 'path', 'query', and 'inner_hits' keys and uses the fully qualified field 'comments.text'. Option A has keys in wrong order (though order is not critical, but inner_hits should be after query), C misses inner_hits, D uses unqualified field 'text'.

🚀 Application

expert

Construct a nested query to find documents with nested comments by 'eve' having likes between 5 and 15

You want to find documents where at least one nested comment has author 'eve' and likes between 5 and 15 inclusive. Which query correctly implements this?

A { "query": { "nested": { "path": "comments", "query": { "match": { "comments.author": "eve" } }, "filter": { "range": { "comments.likes": { "gte": 5, "lte": 15 } } } } } }

B { "query": { "bool": { "must": [ { "match": { "comments.author": "eve" } }, { "range": { "comments.likes": { "gte": 5, "lte": 15 } } } ] } } }

C { "query": { "nested": { "path": "comments", "query": { "bool": { "must": [ { "match": { "comments.author": "eve" } }, { "range": { "comments.likes": { "gte": 5, "lte": 15 } } } ] } } } } }

D { "query": { "nested": { "path": "comments", "query": { "bool": { "should": [ { "match": { "comments.author": "eve" } }, { "range": { "comments.likes": { "gte": 5, "lte": 15 } } } ] } } } } }

💡 Hint

Use a bool must inside nested query to combine conditions on the same nested object.

Option C correctly uses a nested query with a bool must combining match and range conditions on the same nested object. Option C is not nested, so conditions may match different nested objects. Option C uses an invalid 'filter' key inside nested query. Option C uses 'should' which means either condition can match, not both.

Practice

1. What is the main purpose of a nested query in Elasticsearch?

easy

A. To aggregate numeric values across documents

B. To search inside objects stored within other objects

C. To perform full-text search on string fields

D. To update documents in bulk

Solution

1. Step 1: Understand nested objects

   Nested objects are objects stored inside other objects in Elasticsearch documents.

2. Step 2: Purpose of nested query

   The nested query lets you search inside these nested objects specifically.

3. Final Answer:

   To search inside objects stored within other objects -> Option B

4. Quick Check:

   Nested query = search inside nested objects [OK]

Hint: Nested queries search inside nested objects only [OK]

Common Mistakes:

• Confusing nested query with full-text search
• Using nested query on non-nested fields
• Thinking nested query updates documents

2. Which of the following is the correct syntax to specify the path in a nested query?

easy

A. "nested": { "path": "comments", "query": { ... } }

B. "nested": { "field": "comments", "query": { ... } }

C. "nested": { "location": "comments", "query": { ... } }

D. "nested": { "object": "comments", "query": { ... } }

Solution

1. Step 1: Identify the correct key for nested path

   The nested query requires the key path to specify the nested object location.

2. Step 2: Match syntax with options

   Only "nested": { "path": "comments", "query": { ... } } uses "path": "comments" correctly.

3. Final Answer:

   "nested": { "path": "comments", "query": { ... } } -> Option A

4. Quick Check:

   Nested query uses "path" key [OK]

Hint: Use "path" key to specify nested object location [OK]

Common Mistakes:

• Using "field" instead of "path"
• Using incorrect keys like "location" or "object"
• Omitting the "path" key entirely

3. Given this nested mapping for comments and this query:

{
  "query": {
    "nested": {
      "path": "comments",
      "query": {
        "match": { "comments.message": "great" }
      }
    }
  }
}

What documents will this query return?

medium

A. Documents without any comments

B. Documents where all comments contain the word "great"

C. Documents with the word "great" anywhere in the document

D. Documents with at least one comment containing the word "great"

Solution

1. Step 1: Understand nested query behavior

   The nested query matches documents where at least one nested object matches the query condition.

2. Step 2: Analyze the match condition

   The query matches if any comment's message contains "great".

3. Final Answer:

   Documents with at least one comment containing the word "great" -> Option D

4. Quick Check:

   Nested query matches any nested object meeting condition [OK]

Hint: Nested query matches if any nested object matches [OK]

Common Mistakes:

• Thinking all nested objects must match
• Confusing nested query with top-level match
• Ignoring the nested path requirement

4. Identify the error in this nested query:

{
  "query": {
    "nested": {
      "path": "comments",
      "query": {
        "match": { "message": "great" }
      }
    }
  }
}

medium

A. The field "message" should be prefixed with the nested path "comments.message"

B. The "path" key is missing

C. The nested query cannot use match queries

D. The query should be inside a bool query

Solution

1. Step 1: Check field names in nested query

   Fields inside nested queries must be fully qualified with the nested path prefix.

2. Step 2: Identify the missing prefix

   The query uses "message" instead of "comments.message" which is incorrect.

3. Final Answer:

   The field "message" should be prefixed with the nested path "comments.message" -> Option A

4. Quick Check:

   Nested query fields need full path prefix [OK]

Hint: Prefix nested fields with path in queries [OK]

Common Mistakes:

• Omitting nested path prefix in field names
• Assuming nested query disallows match queries
• Forgetting to include the "path" key

5. You have documents with a nested field reviews containing rating and comment. You want to find documents with at least one review having rating >= 4 and comment containing "excellent". Which nested query correctly achieves this?

hard

A. { "bool": { "must": [ { "range": { "reviews.rating": { "gte": 4 } } }, { "match": { "reviews.comment": "excellent" } } ] } }

B. { "nested": { "path": "reviews", "query": { "match": { "reviews.comment": "excellent" } } } }

C. { "nested": { "path": "reviews", "query": { "bool": { "must": [ { "range": { "reviews.rating": { "gte": 4 } } }, { "match": { "reviews.comment": "excellent" } } ] } } } }

D. { "nested": { "path": "reviews", "query": { "range": { "reviews.rating": { "gte": 4 } } } } }

Solution

1. Step 1: Combine conditions inside nested query

   Both rating and comment conditions must be inside the nested query to match the same nested object.

2. Step 2: Use bool must to combine conditions

   Use a bool query with must array to require both conditions.

3. Step 3: Verify field names and structure

   { "nested": { "path": "reviews", "query": { "bool": { "must": [ { "range": { "reviews.rating": { "gte": 4 } } }, { "match": { "reviews.comment": "excellent" } } ] } } } } correctly uses "reviews.rating" and "reviews.comment" inside nested query with bool must.

4. Final Answer:

   { "nested": { "path": "reviews", "query": { "bool": { "must": [ { "range": { "reviews.rating": { "gte": 4 } } }, { "match": { "reviews.comment": "excellent" } } ] } } } } -> Option C

5. Quick Check:

   Combine conditions inside nested bool query [OK]

Hint: Use bool must inside nested query to combine conditions [OK]

Common Mistakes:

• Putting conditions outside nested query
• Using only one condition inside nested query
• Not prefixing fields with nested path