Add parent ID query

Signed-off-by: Fanit Kolchina <[email protected]>

_query-dsl/joining/index.md

@@ -13,9 +13,9 @@ redirect_from:
 OpenSearch is a distributed system in which data is spread across multiple nodes. Thus, running a SQL-like JOIN operation in OpenSearch is resource intensive. As an alternative, OpenSearch provides the following queries that perform join operations and are optimized for scaling across multiple nodes:
 
 - `nested` queries: Act as wrappers for other queries to search [nested]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/nested/) fields. The nested field objects are searched as though they were indexed as separate documents.
-- [`has_child`]({{site.url}}{{site.baseurl}}/query-dsl/joining/has-child/) queries: Search for parent documents whose child documents match the query.
-- [`has_parent`]({{site.url}}{{site.baseurl}}/query-dsl/joining/has-parent/) queries: Search for child documents whose parent documents match the query.
-- `parent_id` queries: A [join]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/join/) field type establishes a parent/child relationship between documents in the same index. `parent_id` queries search for child documents that are joined to a specific parent document. 
+- [`has_child`]({{site.url}}{{site.baseurl}}/query-dsl/joining/has-child/) queries: A [join]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/join/) field type establishes a parent/child relationship between documents in the same index. `has_child` queries search for parent documents whose child documents match the query.
+- [`has_parent`]({{site.url}}{{site.baseurl}}/query-dsl/joining/has-parent/) queries: A [join]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/join/) field type establishes a parent/child relationship between documents in the same index. `has_parent` queries search for child documents whose parent documents match the query.
+- [`parent_id`]({{site.url}}{{site.baseurl}}/query-dsl/joining/parent-id/) queries: A [join]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/join/) field type establishes a parent/child relationship between documents in the same index. `parent_id` queries search for child documents that are joined to a specific parent document. 
 
 If [`search.allow_expensive_queries`]({{site.url}}{{site.baseurl}}/query-dsl/index/#expensive-queries) is set to `false`, then joining queries are not executed.
 {: .important}

_query-dsl/joining/parent-id.md

@@ -0,0 +1,96 @@
+---
+layout: default
+title: Parent ID
+parent: Joining queries
+nav_order: 40
+---
+
+# Parent ID query
+
+The `parent_id` query returns child documents whose parent document has the specified ID. You can establish parent-child relationships between documents in the same index by using a [join]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/join/) field type.
+
+## Example
+
+Before you can run a `parent_id` query, your index must contain a [join]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/join/) field in order to establish parent-child relationships. The index mapping request uses the following format:
+
+```json
+PUT /example_index
+{
+  "mappings": {
+    "properties": {
+      "relationship_field": {
+        "type": "join",
+        "relations": {
+          "parent_doc": "child_doc"
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %} 
+
+For this example, first configure an index that contains documents representing products and their brands as described in the [`has_child` query example]({{site.url}}{{site.baseurl}}/query-dsl/joining/has-child/). 
+
+To search for children documents of a specific parent document, use a `parent_id` query. The following query returns child documents (products) whose parent document has the ID `1`:
+
+```json
+GET testindex1/_search
+{
+  "query": {
+    "parent_id": {
+      "type": "product",
+      "id": "1"
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+The response returns the child product:
+
+```json
+{
+  "took": 57,
+  "timed_out": false,
+  "_shards": {
+    "total": 1,
+    "successful": 1,
+    "skipped": 0,
+    "failed": 0
+  },
+  "hits": {
+    "total": {
+      "value": 1,
+      "relation": "eq"
+    },
+    "max_score": 0.87546873,
+    "hits": [
+      {
+        "_index": "testindex1",
+        "_id": "3",
+        "_score": 0.87546873,
+        "_routing": "1",
+        "_source": {
+          "name": "Mechanical watch",
+          "sales_count": 150,
+          "product_to_brand": {
+            "name": "product",
+            "parent": "1"
+          }
+        }
+      }
+    ]
+  }
+}
+```
+
+## Parameters
+
+The following table lists all top-level parameters supported by `parent_id` queries.
+
+| Parameter  | Required/Optional | Description  |
+|:---|:---|:---|
+| `type` | Required | Specifies the name of the child relationship as defined in the `join` field mapping. |
+| `id` | Required | The ID of the parent document. The query returns child documents associated with this parent document. |
+| `ignore_unmapped` | Optional | Indicates whether to ignore unmapped `type` fields and not return documents instead of throwing an error. You can provide this parameter when querying multiple indexes, some of which may not contain the `type` field. Default is `false`. |