REPLACE operation in AQL

The REPLACE operation removes all attributes of a document and sets the given attributes, excluding immutable system attributes

Each REPLACE operation is restricted to a single collection, and the collection name must not be dynamic. Only a single REPLACE statement per collection is allowed per AQL query, and it cannot be followed by read or write operations that access the same collection, by traversal operations, or AQL functions that can read documents.

You cannot replace the _id, _key, and _rev system attributes, but you can replace the _from and _to attributes.

Replacing a document modifies the document’s revision number (_rev attribute) with a server-generated value.

Syntax

The two syntaxes for a replace operation are:

REPLACE document IN collection
REPLACE keyExpression WITH document IN collection

Both variants can optionally end with an OPTIONS { … } clause.

collection must contain the name of the collection in which the document should be replaced.

document must be an object and contain the attributes and values to set. All existing attributes in the stored document are removed from it and only the provided attributes are set (excluding the immutable _id and _key attributes and the system-managed _rev attribute). This distinguishes the REPLACE from the UPDATE operation, which only affects the attributes you specify in the operation and doesn’t change other attributes of the stored document.

REPLACE <document> IN <collection>

Using the first syntax, the document object must have a _key attribute with the document key. The existing document with this key is replaced with the attributes provided by the document object (except for the _id, _key, and _rev system attributes).

The following query replaces the document identified by the key my_key in the users collection, only setting a name and a status attribute. The key is passed via the _key attribute alongside other attributes:

REPLACE { _key: "my_key", name: "Jon", status: "active" } IN users

The following query is invalid because the object does not contain a _key attribute and thus it is not possible to determine the document to be replaced:

REPLACE { name: "Jon" } IN users

You can combine the REPLACE operation with a FOR loop to determine the necessary key attributes, like shown below:

FOR u IN users
  REPLACE { _key: u._key, name: CONCAT(u.firstName, " ", u.lastName), status: u.status } IN users

Note that the REPLACE and FOR operations are independent of each other and u does not automatically define a document for the REPLACE statement. Thus, the following query is invalid:

FOR u IN users
  REPLACE { name: CONCAT(u.firstName, " ", u.lastName), status: u.status } IN users

REPLACE <keyExpression> WITH <document> IN <collection>

Using the second syntax, the document to replace is defined by the keyExpression. It can either be a string with the document key, an object which contains a _key attribute with the document key, or an expression that evaluates to either of these two. The existing document with this key is replaced with the attributes provided by the document object (except for the _id, _key, and _rev system attributes).

The following query replaces the document identified by the key my_key in the users collection, only setting a name and a status attribute. The key is passed as a string in the keyExpression. The attributes to set are passed separately as the document object:

REPLACE "my_key" WITH { name: "Jon", status: "active" } IN users

The document object may contain a _key attribute, but it is ignored.

You cannot define the document to replace using an _id attribute, nor pass a document identifier as a string (like "users/john"). However, you can use PARSE_IDENTIFIER(<id>).key as keyExpression to get the document key as a string:

LET key = PARSE_IDENTIFIER("users/john").key
REPLACE key WITH { ... } IN users

Comparison of the syntaxes

Both syntaxes of the REPLACE operation allow you to define the document to modify and the attributes to set. The document to update is effectively identified by a document key in combination with the specified collection.

The REPLACE operation supports different ways of specifying the document key. You can choose the syntax variant that is the most convenient for you.

The following queries are equivalent:

FOR u IN users
  REPLACE u WITH { name: CONCAT(u.firstName, " ", u.lastName), status: u.status } IN users
FOR u IN users
  REPLACE u._key WITH { name: CONCAT(u.firstName, " ", u.lastName), status: u.status } IN users
FOR u IN users
  REPLACE { _key: u._key } WITH { name: CONCAT(u.firstName, " ", u.lastName), status: u.status } IN users
FOR u IN users
  REPLACE { _key: u._key, name: CONCAT(u.firstName, " ", u.lastName), status: u.status } IN users

Dynamic key expressions

A REPLACE operation may replace arbitrary documents, using either of the two syntaxes:

FOR i IN 1..1000
  REPLACE { _key: CONCAT("test", i), name: "Paula", status: "active" } IN users
FOR i IN 1..1000
  REPLACE CONCAT("test", i) WITH { name: "Paula", status: "active" } IN users

Target a different collection

The documents a REPLACE operation modifies can be in a different collection than the ones produced by a preceding FOR operation:

FOR u IN users
  FILTER u.active == false
  REPLACE u WITH { status: "inactive", name: u.name } IN backup

Note how documents are read from the users collection but replaced in another collection called backup. Both collections need to use matching document keys for this to work.

Although the u variable holds a whole document, it is only used to define the target document. The _key attribute of the object is extracted and the target document is solely defined by the document key string value and the specified collection of the REPLACE operation (backup). There is no link to the original collection (users).

Query options

You can optionally set query options for the REPLACE operation:

REPLACE ... IN users OPTIONS { ... }

ignoreErrors

You can use ignoreErrors to suppress query errors that may occur when trying to replace non-existing documents or when violating unique key constraints:

FOR i IN 1..1000
  REPLACE CONCAT("test", i)
  WITH { foobar: true } IN users
  OPTIONS { ignoreErrors: true }

You cannot modify the _id, _key, and _rev system attributes, but attempts to change them are ignored and not considered errors.

waitForSync

To make sure data are durable when a replace query returns, there is the waitForSync query option:

FOR i IN 1..1000
  REPLACE CONCAT("test", i)
  WITH { foobar: true } IN users
  OPTIONS { waitForSync: true }

ignoreRevs

In order to not accidentally overwrite documents that have been modified since you last fetched them, you can use the option ignoreRevs to either let ArangoDB compare the _rev value and only succeed if they still match, or let ArangoDB ignore them (default):

FOR i IN 1..1000
  REPLACE { _key: CONCAT("test", i), _rev: "1287623" }
  WITH { foobar: true } IN users
  OPTIONS { ignoreRevs: false }

exclusive

The RocksDB engine does not require collection-level locks. Different write operations on the same collection do not block each other, as long as there are no write-write conflicts on the same documents. From an application development perspective it can be desired to have exclusive write access on collections, to simplify the development. Note that writes do not block reads in RocksDB. Exclusive access can also speed up modification queries, because we avoid conflict checks.

Use the exclusive option to achieve this effect on a per query basis:

FOR doc IN collection
  REPLACE doc
  WITH { replaced: true } IN collection
  OPTIONS { exclusive: true }

refillIndexCaches

Whether to update existing entries in in-memory index caches if documents replacements affect the edge index or cache-enabled persistent indexes.

REPLACE { _key: "123", _from: "vert/C", _to: "vert/D" } IN edgeColl
  OPTIONS { refillIndexCaches: true }

versionAttribute

You can use the versionAttribute option for external versioning support. If set, the attribute with the name specified by the option is looked up in the stored document and the attribute value is compared numerically to the value of the versioning attribute in the supplied document that is supposed to replace it.

If the version number in the new document is higher (rounded down to a whole number) than in the document that already exists in the database, then the replace operation is performed normally. This is also the case if the new versioning attribute has a non-numeric value, if it is a negative number, or if the attribute doesn’t exist in the supplied or stored document.

If the version number in the new document is lower or equal to what exists in the database, the operation is not performed and the existing document thus not changed. No error is returned in this case.

The attribute can only be a top-level attribute.

For example, the following query conditionally replaces an existing document with the key "123" if the attribute externalVersion currently has a value below 5:

REPLACE { _key: "123", externalVersion: 5, anotherAttribute: true } IN coll
  OPTIONS { versionAttribute: "externalVersion" }

You can check if OLD._rev and NEW._rev are different to determine if the document has been changed.

Returning the modified documents

You can optionally return the documents modified by the query. In this case, the REPLACE operation needs to be followed by a RETURN operation. Intermediate LET operations are allowed, too. These operations can refer to the pseudo-variables OLD and NEW. The OLD pseudo-variable refers to the document revisions before the replace, and NEW refers to the document revisions after the replace.

Both OLD and NEW contain all document attributes, even those not specified in the replace expression.

REPLACE document IN collection options RETURN OLD
REPLACE document IN collection options RETURN NEW
REPLACE keyExpression WITH document IN collection options RETURN OLD
REPLACE keyExpression WITH document IN collection options RETURN NEW

Following is an example using a variable named previous to return the original documents before modification. For each replaced document, the document key is returned:

FOR u IN users
  REPLACE u WITH { value: "test" } IN users
  LET previous = OLD
  RETURN previous._key

The following query uses the NEW pseudo-value to return the replaced documents, without some of their system attributes:

FOR u IN users
  REPLACE u WITH { value: "test" } IN users
  LET replaced = NEW
  RETURN UNSET(replaced, "_key", "_id", "_rev")

Transactionality

On a single server, replace operations are executed transactionally in an all-or-nothing fashion.

A query may execute intermediate transaction commits in case the running transaction (AQL query) hits the specified size thresholds. In this case, the query’s operations carried out so far are committed and not rolled back in case of a later abort/rollback. This behavior can be controlled by adjusting the intermediate commit settings for the RocksDB engine. See Known limitations for AQL queries.

For sharded collections, the entire query and/or replace operation may not be transactional, especially if it involves different shards and/or DB-Servers.

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