Nested queries for nested objects in Elasticsearch - Step-by-Step Execution

Concept Flow - Nested queries for nested objects

Start Query

↓

Identify nested field

↓

Build nested query block

↓

Specify inner query conditions

↓

Execute nested query

↓

Return matching documents

The query starts by identifying the nested field, builds a nested query block with inner conditions, executes it, and returns matching documents.

Execution Sample

Elasticsearch

{
  "query": {
    "nested": {
      "path": "comments",
      "query": {
        "match": { "comments.author": "Alice" }
      }
    }
  }
}

This query searches documents where the nested 'comments' array has an author named 'Alice'.

Execution Table

Step	Action	Query Part	Effect	Result
1	Start query	Full query JSON	Identify 'comments' as nested path	Prepare nested query context
2	Build nested query	"nested" block with path 'comments'	Focus search inside 'comments' nested objects	Nested query ready
3	Specify inner query	"match" on 'comments.author' = 'Alice'	Filter nested objects where author is Alice	Inner query set
4	Execute nested query	Run nested query on documents	Search nested 'comments' arrays for matches	Documents with matching nested objects found
5	Return results	Matched documents	Documents containing at least one comment by Alice	Final matched documents returned

💡 Query completes after returning documents matching nested 'comments.author' = 'Alice'

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
nested_path	undefined	"comments"	"comments"	"comments"	"comments"
inner_query	undefined	undefined	{"match": {"comments.author": "Alice"}}	{"match": {"comments.author": "Alice"}}	{"match": {"comments.author": "Alice"}}
matched_documents	empty	empty	empty	documents with comments.author='Alice'	documents with comments.author='Alice'

Key Moments - 3 Insights

Why do we need to specify the 'path' in the nested query?

The 'path' tells Elasticsearch which nested field to search inside. Without it, the query won't know where to look for nested objects. See execution_table step 2.

Can we match nested fields without using a nested query?

No, because nested fields are stored separately internally. A nested query is required to correctly search inside nested objects. See execution_table step 3 and 4.

What happens if no nested objects match the inner query?

The document will not be returned because the nested query requires at least one nested object to match. See execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the nested path specified?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Check the 'Query Part' column in execution_table rows.

According to variable_tracker, what is the value of 'matched_documents' after step 4?

A documents with comments.author='Alice'

B empty

C undefined

D all documents

💡 Hint

Look at the 'matched_documents' row and the 'After Step 4' column.

If we remove the 'nested' block and only use the inner 'match' query, what will happen?

A The query will work the same

B The query will not correctly match nested objects

C The query will fail with syntax error

D The query will return all documents

💡 Hint

Refer to key_moments about why nested queries are needed.

Concept Snapshot

Nested queries let you search inside nested objects.
Use the 'nested' query with a 'path' to specify the nested field.
Inside, put your query for nested fields.
Elasticsearch matches documents with nested objects meeting the inner query.
Without 'nested', nested fields won't match correctly.

Full Transcript

This visual execution shows how a nested query works in Elasticsearch. The query starts by identifying the nested field 'comments'. Then it builds a nested query block specifying the path 'comments'. Inside this block, it adds a match query to find nested objects where 'comments.author' equals 'Alice'. The query runs and searches inside the nested 'comments' arrays of documents. It returns documents that have at least one comment by Alice. Variables like 'nested_path' and 'inner_query' are set early, and 'matched_documents' updates after execution. Key points include the importance of specifying the nested path and why nested queries are necessary to search nested objects correctly. The quiz tests understanding of these steps and variable states.

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