Nested queries for nested objects in Elasticsearch - Mini Project: Build & Apply

Nested queries for nested objects

📖 Scenario: You work with a bookstore database in Elasticsearch. Each book document has a nested field called authors that stores multiple authors with their name and age. You want to find books where at least one author is younger than 40.

🎯 Goal: Build an Elasticsearch query using nested queries to find books with authors younger than 40.

📋 What You'll Learn

Create an index mapping with a nested authors field

Add a config variable for the age threshold

Write a nested query to find books with authors younger than the threshold

Print the final query JSON

💡 Why This Matters

🌍 Real World

Nested queries are used when you have complex data with arrays of objects, like books with multiple authors, and you want to search inside those nested objects.

💼 Career

Many jobs working with Elasticsearch require building nested queries to filter or search data accurately inside nested fields.

1

Create index mapping with nested authors

Create a variable called mapping that holds the Elasticsearch index mapping JSON. It should define a books index with a authors field of type nested. Each author has name (text) and age (integer).

Elasticsearch

# Create the mapping variable with nested authors field
# Your code here

Hint

Use a dictionary with mappings and properties. The authors field must have "type": "nested".

2

Set age threshold variable

Create a variable called age_threshold and set it to 40. This will be used to find authors younger than this age.

Elasticsearch

mapping = {
    "mappings": {
        "properties": {
            "title": {"type": "text"},
            "authors": {
                "type": "nested",
                "properties": {
                    "name": {"type": "text"},
                    "age": {"type": "integer"}
                }
            }
        }
    }
}

# Create age_threshold variable and set to 40
# Your code here

Hint

Just create a variable age_threshold and assign the number 40.

3

Write nested query for authors younger than threshold

Create a variable called query that holds an Elasticsearch nested query. It should search the authors nested field for authors with age less than age_threshold. Use nested query with path set to authors and a range query on authors.age with lt set to age_threshold.

Elasticsearch

mapping = {
    "mappings": {
        "properties": {
            "title": {"type": "text"},
            "authors": {
                "type": "nested",
                "properties": {
                    "name": {"type": "text"},
                    "age": {"type": "integer"}
                }
            }
        }
    }
}
age_threshold = 40

# Create the nested query in variable 'query' to find authors younger than age_threshold
# Your code here

Hint

Use a dictionary with query key, inside it a nested query with path and a range query on authors.age.

4

Print the final query JSON

Write a print statement to display the query variable.

Elasticsearch

mapping = {
    "mappings": {
        "properties": {
            "title": {"type": "text"},
            "authors": {
                "type": "nested",
                "properties": {
                    "name": {"type": "text"},
                    "age": {"type": "integer"}
                }
            }
        }
    }
}
age_threshold = 40

query = {
    "query": {
        "nested": {
            "path": "authors",
            "query": {
                "range": {
                    "authors.age": {
                        "lt": age_threshold
                    }
                }
            }
        }
    }
}

# Print the query variable
# Your code here

Hint

Use print(query) to show the query dictionary.

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