Sub-Document Operations

  • how-to
    Sub-Document operations can be used to efficiently access and change parts of documents.

    Sub-Document operations may be quicker and more network-efficient than full-document operations such as Upsert, Replace and Get because they only transmit the accessed sections of the document over the network.

    Sub-Document operations are also atomic, in that if one Sub-Document mutation fails then all will, allowing safe modifications to documents with built-in concurrency control.

    The complete code sample used on this page can be downloaded from the GitHub repo for the Python docs, from which you can see in context how to authenticate and connect to a Couchbase Cluster, then perform these sub-document operations.


    Starting with Couchbase Server 4.5 you can atomically and efficiently update and retrieve parts of a document.

    These parts are called Sub-Documents.

    While full-document retrievals retrieve the entire document and full document updates require sending the entire document, Sub-Document retrievals only retrieve relevant parts of a document and Sub-Document updates only require sending the updated portions of a document.

    You should use Sub-Document operations when you are modifying only portions of a document, and full-document operations when the contents of a document is to change significantly.

    The Sub-Document operations described on this page are for Key-Value requests only: they are not related to Sub-Document SQL++ (formerly N1QL) queries. (Sub-Document SQL++ queries are explained in the section Querying with SQL++.)

    In order to use Sub-Document operations you need to specify a path indicating the location of the Sub-Document. The path follows SQL++ syntax. Considering the document:

      "name": "Douglas Reynholm",
      "email": "",
      "addresses": {
        "billing": {
          "line1": "123 Any Street",
          "line2": "Anytown",
          "country": "United Kingdom"
        "delivery": {
          "line1": "123 Any Street",
          "line2": "Anytown",
          "country": "United Kingdom"
      "purchases": {
        "complete": [
          339, 976, 442, 666
        "abandoned": [
          157, 42, 999

    The paths name, and purchases.complete[0] are all valid paths.


    The lookup_in method expects an Iterable of Sub-Document Spec (see API reference doc). The examples below show how either a list or a tuple might be used. If using a tuple with only a single Sub-Document Spec be sure to include the trailing comma.

    The lookup_in operations query the document for certain path(s); these path(s) are then returned. You have a choice of actually retrieving the document path using the get Sub-Document operation, or simply querying the existence of the path using the exists Sub-Document operation. The latter saves even more bandwidth by not retrieving the contents of the path if it is not needed.

    Retrieve sub-document value
    result = collection.lookup_in('customer123',
    country = result.content_as[str](0)  # 'United Kingdom'
    Check existence of sub-document path
    result = collection.lookup_in('customer123', [SD.exists('purchases.pending[-1]')])
    print(f'Path exists: {result.exists(0)}.')
    # Path exists:  False.

    Multiple operations can be combined:

    Combine multiple lookup operations
    result = collection.lookup_in('customer123',[SD.get(''),
    print('Path exists: {}.'.format(result.exists(1)))
    # path exists: True.


    The mutate_in method expects an Iterable of Sub-Document Spec (see API reference doc). The examples below show how either a list or a tuple might be used. If using a tuple with only a single Sub-Document Spec be sure to include the trailing comma.

    Mutation operations modify one or more paths in the document. The simplest of these operations is upsert, which, similar to the fulldoc-level upsert, will either modify the value of an existing path or create it if it does not exist:

    Upserting a new sub-document
    collection.mutate_in('customer123', [SD.upsert('fax', '311-555-0151')])

    Likewise, the insert operation will only add the new value to the path if it does not exist:

    Inserting a sub-document
    collection.mutate_in('customer123', [SD.insert('purchases.pending', [42, True, 'None'])])
        collection.mutate_in('customer123', [SD.insert('purchases.complete',[42, True, 'None'])])
    except PathExistsException:
        print('Path exists, cannot use insert.')

    Dictionary values can also be replaced or removed, and you may combine any number of mutation operations within the same general mutate_in API. Here’s an example of one which replaces one path and removes another.

    mutateIn is an atomic operation. If any single ops fails, then the entire document is left unchanged.

    Array append and prepend

    The array_prepend and array_append operations are true array prepend and append operations. Unlike fulldoc append/prepend operations (which simply concatenate bytes to the existing value), array_append and array_prepend are JSON-aware:

    collection.mutate_in('customer123', (SD.array_append('purchases.complete', 777),))
    # purchases.complete is now [339, 976, 442, 666, 777]
    collection.mutate_in('customer123', [SD.array_prepend('purchases.abandoned', 18)])
    # purchases.abandoned is now [18, 157, 42, 999]

    If your document only needs to contain an array, you do not have to create a top-level object wrapper to contain it. Simply initialize the document with an empty array and then use the empty path for subsequent Sub-Document array operations:

    Creating and populating an array document
    collection.upsert('my_array', [])
    collection.mutate_in('my_array', [SD.array_append('', 'some element')])
    # the document my_array is now ['some element']

    If you wish to add multiple values to an array, you may do so by passing multiple values to the array_append, array_prepend, or array_insert operations. Be sure to know the difference between passing a collection of multiple elements (in which case the collection is inserted as a single element in the array, as a sub-array) and passing multiple elements (in which case the elements are appended individually to the array):

    Add multiple elements to an array
    collection.mutate_in('my_array', [SD.array_append('', 'elem1', 'elem2', 'elem3')])
    # the document my_array is now ['some_element', 'elem1', 'elem2', 'elem3']
    Add single array as element to existing array
    collection.mutate_in('my_array', [SD.array_append('', ['elem4', 'elem5', 'elem6'])])
    # the document my_array is now ['some_element', 'elem1', 'elem2', 'elem3',
    #                                   ['elem4', 'elem5', 'elem6']]]

    Note that passing multiple values to a single array_append operation results in greater performance increase and bandwidth savings than simply specifying a single array_append for each element.

    Adding multiple elements to array (slow)
    collection.mutate_in('my_array', (SD.array_append('', 'elem7'),
                                      SD.array_append('', 'elem8'),
                                      SD.array_append('', 'elem9')))

    If you wish to create an array if it does not exist and also push elements to it within the same operation you may use the create_parents option:

    collection.upsert('some_doc', {})
    collection.mutate_in('some_doc', [SD.array_prepend('some.array',

    Arrays as Unique Sets

    Limited support also exists for treating arrays like unique sets, using the array_addunique command. This will do a check to determine if the given value exists or not before actually adding the item to the array:

        collection.mutate_in('customer123', [SD.array_addunique('purchases.complete', 95)])
    except PathExistsException:
        print('Path already exists.')
        collection.mutate_in('customer123', [SD.array_addunique('purchases.complete', 95)])
    except PathExistsException:
        print('Path already exists.')

    Note that currently the array_addunique will fail with a SubdocPathMismatchException if the array contains JSON floats, objects, or arrays. The array_addunique operation will also fail with SubdocCantInsertValueException if the value to be added is one of those types as well.

    Note that the actual position of the new element is undefined, and that the array is not ordered.

    Array insertion

    New elements can also be inserted into an array. While append will place a new item at the end of an array and prepend will place it at the beginning, insert allows an element to be inserted at a specific position. The position is indicated by the last path component, which should be an array index. For example, to insert "cruel" as the second element in the array ["Hello", "world"], the code would look like:

    collection.upsert('array', [])
    collection.mutate_in('array', [SD.array_append('', 'hello', 'world')])
    collection.mutate_in('array', [SD.array_insert('[1]', 'cruel')])

    Note that the array must already exist and that the index must be valid (i.e. it must not point to an element which is out of bounds).

    Counters and Numeric Fields

    Counter operations allow the manipulation of a numeric value inside a document. These operations are logically similar to the counter operation on an entire document:

    result = collection.mutate_in('customer123', (SD.counter('logins', 1),))
    num_logins = collection.get('customer123').content_as[dict]['logins']
    print(f'Number of logins: {num_logins}.')
    # Number of logins: 1.

    The increment and decrement operations perform simple arithmetic against a numeric value. The updated value is returned.

    player_key = 'player432'
    collection.upsert(player_key, {'gold': 1000})
    collection.mutate_in(player_key, (SD.counter('gold', -150),))
    result = collection.lookup_in(player_key, (SD.get('gold'),))
    print(f'{player_key} has {result.content_as[int](0)} gold remaining.')
    # player432 has 850 gold remaining.

    The existing value for counter operations must be within range of a 64 bit signed integer. If the value does not exist, the operation will create it (and its parents, if create_path is enabled).

    Note that there are several differences as compared to the full-document counter operations:

    • Sub-Document counters have a range of -9223372036854775807 to 9223372036854775807, whereas full-document counters have a range of 0 to 18446744073709551615

    • Sub-Document counter operations protect against overflow and underflow, returning an error if the operation would exceed the range. Full-document counters will use normal C semantics for overflow (in which the overflow value is carried over above 0), and will silently fail on underflow, setting the value to 0 instead.

    • Sub-Document counter operations can operate on any numeric value within a document, while full-document counter operations require a specially formatted counter document with only the counter value.

    Executing Multiple Operations

    Multiple Sub-Document operations can be executed at once on the same document, allowing you to retrieve or modify several Sub-Documents at once. When multiple operations are submitted within the context of a single lookup_in or mutate_in command, the server will execute all the operations with the same version of the document.

    Unlike batched operations which is simply a way of sending multiple individual operations efficiently on the network, multiple Sub-Document operations are formed into a single command packet, which is then executed atomically on the server. You can submit up to 16 operations at a time.

    When submitting multiple mutation operations within a single mutate_in command, those operations are considered to be part of a single transaction: if any of the mutation operations fail, the server will logically roll-back any other mutation operations performed within the mutate_in, even if those commands would have been successful had another command not failed.

    When submitting multiple retrieval operations within a single lookup_in command, the status of each command does not affect any other command. This means that it is possible for some retrieval operations to succeed and others to fail. While their statuses are independent of each other, you should note that operations submitted within a single lookup_in are all executed against the same version of the document.

    Creating Paths

    Sub-Document mutation operations such as upsert or insert will fail if the immediate parent is not present in the document. Consider:

        "level_0": {
            "level_1": {
                "level_2": {
                    "level_3": {
                        "some_field": "some_value"

    Looking at the some_field field (which is really level_0.level_1.level_2.level_3.some_field), its immediate parent is level_3. If we were to attempt to insert another field, level_0.level_1.level_2.level_3.another_field, it would succeed because the immediate parent is present. However if we were to attempt to insert to it would fail, because (which would be the immediate parent) does not exist. Attempting to perform such an operation would result in a Path Not Found error.

    By default the automatic creation of parents is disabled, as a simple typo in application code can result in a rather confusing document structure. Sometimes it is necessary to have the server create the hierarchy however. In this case, the create_path option may be used.

    collection.mutate_in('customer123', [SD.upsert('',
                                                   ), create_parents=True)])

    Reading Sub-Documents From Replicas

    Couchbase Server 7.6 and later support Sub-Doc lookup from replicas.

    The collection.lookup_in_any_replica() method returns the first response — from active or replica:

        result = collection.lookup_in_any_replica('customer123',
        country = result.content_as[str](0)  # 'United Kingdom'
        print(f'Is result replica={result.is_replica}')
    except PathNotFoundException as ex:
        print(('The version of the document on the server node '
               'that responded quickest did not have the requested '
    except DocumentUnretrievableException as ex:
        print('Document not present on any server node.')

    The collection.lookup_in_all_replicas() fetches all available replicas (and the active copy), and returns all responses.

    result = collection.lookup_in_all_replicas('customer123',
    for res in result:
            country = res.content_as[str](0)  # 'United Kingdom'
            print(f'Is result replica={res.is_replica}')
        except PathNotFoundException as ex:
            print(('The version of the document on one of the server nodes '
                   'did not have the requested field.'))

    You may want to use lookup_in_all_replicas to build a consensus, but it’s more likely that you’ll make use of lookup_in_any_replica as a fallback to a lookupIn, when the active node times out.

    Concurrent Modifications

    Concurrent Sub-Document operations on different parts of a document will not conflict. For example the following two blocks can execute concurrently without any risk of conflict:

    collection.mutate_in('customer123', [SD.array_append('purchases.complete', 998)])
    collection.mutate_in('customer123', [SD.array_append('purchases.complete', 999)])

    Even when modifying the same part of the document, operations will not necessarily conflict. For example, two concurrent array_append operations to the same array will both succeed, never overwriting the other.

    So in some cases the application will not need to supply a CAS value to protect against concurrent modifications.

    If CAS is required then it can be provided like this:

                         [SD.array_append('purchases.complete', 999)],


    Couchbase’s traditional 'client verified' durability, using PersistTo and ReplicateTo, is still available, particularly for talking to Couchbase Server 6.0 and earlier. Refer to the API docs for the legacy API.

                         [SD.insert('username', 'dreynholm')],

    In Couchbase Server 6.5 and up, this is built upon with Durable Writes, which uses the concept of majority to indicate the number of configured Data Service nodes to which commitment is required:

                         [SD.insert('username', 'dreynholm')],

    Error Handling

    Sub-Document operations have their own set of errors. When programming with Sub-Document, be prepared for any of the full-document errors (such as DocumentDoesNotExistException) as well as special Sub-Document errors which are received when certain constraints are not satisfied. Some of the errors include:

    • PathNotFoundException: When retrieving a path, this means the path does not exist in the document. When inserting or upserting a path, this means the immediate parent does not exist.

    • PathExistsException: In the context of an insert, it means the given path already exists. In the context of arrayAddUnique, it means the given value already exists.

    • PathMismatchException: This means the path may exist in the document, but that there is a type conflict between the path in the document and the path in the command. Consider the document:

      { "tags": ["reno", "nevada", "west", "sierra"] }

      The path tags.sierra is a mismatch, since tags is actually an array, while the path assumes it is a JSON object (dictionary).

    • DocumentNotJsonException: This means you are attempting to modify a binary document using Sub-Document operations.

    • PathInvalidException: This means the path is invalid for the command. Certain commands such as arrayInsert expect array elements as their final component, while others such as upsert and insert expect dictionary (object) keys.

    If a Sub-Document command fails a top-level error is reported (MultiMutationException), rather than an individual error code (e.g. PathNotFoundException). When receiving a top-level error code, you should traverse the results of the command to see which individual code failed.

    Path Syntax

    Path syntax largely follows SQL++ conventions: A path is divided into components, with each component referencing a specific level in a document hierarchy. Components are separated by dots (.) in the case where the element left of the dot is a dictionary, or by brackets ([n]) where the element left of the bracket is an array and n is the index within the array.

    As a special extension, you can indicate the last element of an array by using an index of -1, for example to get the last element of the array in the document


    Use some.array[-1] as the path, which will return the element 0.

    Each path component must conform as a JSON string, as if it were surrounded by quotes, and any character in the path which may invalidate it as a JSON string must be escaped by a backslash (\). In other words, the path component must match exactly the path inside the document itself. For example:

    {"literal\"quote": {"array": []}}

    must be referenced as literal\"quote.array.

    If the path also has special path characters (i.e. a dot or brackets) it may be escaped using SQL++ escapes. Considering the document

    {"literal[]bracket": {"": true}}

    A path such as `literal[]bracket`.``. You can use double-backticks (``) to reference a literal backtick.

    If you need to combine both JSON and path-syntax literals you can do so by escaping the component from any JSON string characters (e.g. a quote or backslash) and then encapsulating it in backticks (`path`).

    Currently, paths cannot exceed 1024 characters, and cannot be more than 32 levels deep.

    Extended Attributes

    Extended Attributes (also known as XATTRs), built upon the Sub-Document API, allow developers to define application-specific metadata that will only be visible to those applications that request it or attempt to modify it. This might be, for example, meta-data specific to a programming framework that should be hidden by default from other frameworks or libraries, or possibly from other versions of the same framework. They are not intended for use in general applications, and data stored there cannot be accessed easily by some Couchbase services, such as Search.