Sub-Document Operations with the Ruby SDK

how-to

Sub-document operations can be used to efficiently access 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, allowing safe modifications to documents with built-in concurrency control.

Sub-documents

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 N1QL queries. (Sub-document N1QL queries are explained in the section Querying with N1QL.)

In order to use sub-document operations you need to specify a path indicating the location of the sub-document. The path follows N1QL syntax below^, and N1QL Queries and Results). Considering the document:

customer123.json

{
  "name": "Douglas Reynholm",
  "email": "douglas@reynholmindustries.com",
  "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, addresses.billing.country and purchases.complete[0] are all valid paths.

Retrieving

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 subdoc-get sub-document operation, or simply querying the existence of the path using the subdoc-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", [
    LookupInSpec.get("addresses.delivery.country")
])
country = result.content(0)
puts "Country = #{country}"
#=> Country = United Kingdom

Check existence of sub-document path

result = collection.lookup_in("customer123", [
    LookupInSpec.exists("addresses.delivery.does_not_exist")
])
puts "exists: #{result.exists?(0)}"
#=> exists: false

Multiple operations can be combined as well:

Combine multiple lookup operations

result = collection.lookup_in("customer123", [
    LookupInSpec.get("addresses.delivery.country"),
    LookupInSpec.exists("addresses.delivery.does_not_exist")
])
puts "Country = #{result.content(0)}"
#=> Country = United Kingdom
puts "exists: #{result.exists?(1)}"
#=> exists: false

Mutating

Mutation operations modify one or more paths in the document. The simplest of these operations is subdoc-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

result = collection.mutate_in("customer123", [
    MutateInSpec.upsert("email", "dougr96@hotmail.com")
])
puts "CAS: #{result.cas}"
#=> CAS: 3769293415702

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

Inserting a sub-document

begin
  collection.mutate_in("customer123", [
      MutateInSpec.insert("email", "dougr96@hotmail.com")
  ])
rescue Error::PathExists
  puts "Error, path already exists"
end

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.

result = collection.mutate_in("customer123", [
    MutateInSpec.remove("addresses.billing"),
    MutateInSpec.replace("email", "dougr96@hotmail.com")
])
puts "success: #{result.success?}, CAS: #{result.cas}"
#=> success: true, CAS: 149588684383510

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

Array append and prepend

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

collection.mutate_in("customer123", [
    MutateInSpec.array_append("purchases.complete", [777])
])
complete_purchases = collection.get("customer123").content["purchases"]["complete"]
puts "purchases.complete: #{complete_purchases}"
#=> purchases.complete: [339, 976, 442, 666, 777]

collection.mutate_in("customer123", [
    MutateInSpec.array_prepend("purchases.abandoned", [18])
])
abandoned_purchases = collection.get("customer123").content["purchases"]["abandoned"]
puts "purchases.abandoned: #{abandoned_purchases}"
#=> purchases.abandoned: [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", [
    MutateInSpec.array_append("", ["some element"])
])
puts "my_array: #{collection.get("my_array").content}"
#=> my_array: ["some element"]

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-path option:

collection.upsert("some_doc", {})

collection.mutate_in("some_doc", [
    MutateInSpec.array_append("some.array", ["hello world"]).create_path
])
puts "some_doc: #{collection.get("some_doc").content}"
#=> some_doc: {"some"=>{"array"=>["hello world"]}}

Arrays as Unique Sets

Limited support also exists for treating arrays like unique sets, using the .array_add_unique() method. 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", [
    MutateInSpec.array_add_unique("purchases.complete", 95)
])

begin
  collection.mutate_in("customer123", [
      MutateInSpec.array_add_unique("purchases.complete", 95)
  ])
rescue Error::PathExists
  puts "Error, value already exists in the path"
end

Note that currently the .array_add_unique() will fail with a Path Mismatch error if the array contains JSON floats, objects, or arrays. The .array_add_unique() operation will also fail with Cannot Insert 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("some_doc", {
    "foo" => {
        "bar" => ["hello", "world"]
    }
})

collection.mutate_in("some_doc", [
    MutateInSpec.array_insert("foo.bar[1]", ["cruel"])
])
puts "foo.bar: #{collection.get("some_doc").content["foo"]["bar"]}"
#=> foo.bar: ["hello", "cruel", "world"]

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", [
    MutateInSpec.increment("logins", 1)
])

# Counter operations return the updated count
puts "current logins: #{result.content(0)}"
#=> current logins: 1

The subdoc-counter operation performs simple arithmetic against a numeric value, either incrementing or decrementing the existing value. The new value is returned.

collection.upsert("player432", {"gold" => 1000})

result = collection.mutate_in("player432", [
    MutateInSpec.decrement("gold", 150)
])

# Counter operations return the updated count
puts "player432 has #{result.content(0)} gold coins"
#=> player432 has 850 gold coins

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

Note that there are several differences between subdoc-counter and the full-document counter operations:

Sub-document counters have a range of -9223372036854775807 to 9223372036854775807 (i.e. Iint64.MinValue and Int64.MaxValue), whereas full-document counters have a range of 0 to 18446744073709551615 (`UInt64.MaxValue)
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 subdoc 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 parents

Sub-document mutation operations such as subdoc-upsert or subdoc-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 subdoc-insert to level_1.level_2.foo.bar it would fail, because level_1.level_2.foo (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.upsert("some_doc", {})

collection.mutate_in("some_doc", [
    MutateInSpec
        .upsert("level_0.level_1.foo.bar.phone",
                {"num" => "0118 999 881 999 119 725 3"})
        .create_path
])

puts "some_doc: #{collection.get("some_doc").content}"
#=> some_doc: {"level_0"=>{"level_1"=>{"foo"=>{"bar"=>{"phone"=>{"num"=>"0118 999 881 999 119 725 3"}}}}}}

CAS Semantics

Subdoc mostly eliminates the need for tracking the CAS value. Subdoc operations are atomic and therefore if two different threads access two different sub-documents then no conflict will arise.

get_result = collection.get("player432")

options = Collection::MutateInOptions.new
options.cas = get_result.cas
collection.mutate_in("player432", [
    MutateInSpec.decrement("gold", 150)
], options)

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

While CAS is no longer so strongly required to ensure document updates are preserved, as Sub-Doc reduces the chance of losing a mutation, it may still be needed to ensure document state remains consistent over multiple invocations of mutate-in: Sometimes it’s important to ensure the entire document didn’t change state since the last operation, such as in the case subdoc-remove operations to ensure that the element being removed was not already replaced by something else.

Durability

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:

options = Collection::MutateInOptions.new
options.durability_level = :majority
collection.mutate_in("some_doc", [
    MutateInSpec.insert("foo", "bar")
])

Error handling

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

Path does not exist: 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.
Path already exists: In the context of an insert, it means the given path already exists. In the context of array-add-unique, it means the given value already exists.
Path mismatch: 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).
Document not JSON: This means you are attempting to modify a binary document using sub-document operations.
Invalid path: This means the path is invalid for the command. Certain commands such as subdoc-array-insert expect array elements as their final component, while others such as subdoc-upsert and subdoc-insert expect dictionary (object) keys.

If a Sub-Document command fails a top-level error is reported (Multi Command Failure), rather than an individual error code (e.g. Path Not Found). 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 N1QL 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

{"some":{"array":[1,2,3,4,5,6,7,8,9,0]}}

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 N1QL escapes. Considering the document

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

A path such as `literal[]bracket`.`literal.dot`. 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.

XDCR

XDCR only replicates full documents. Sub-documents are only replicated as part of the full document.