REPLACE

The REPLACE keyword can be used to completely replace documents in a collection. On a single server, the replace operation is executed transactionally in an all-or-nothing fashion.

If the RocksDB engine is used and intermediate commits are enabled, 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 will be committed and not rolled back in case of a later abort/rollback. That behavior can be controlled by adjusting the intermediate commit settings for the RocksDB engine.

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

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. The system attributes _id, _key and _rev cannot be replaced, _from and _to can.

The two syntaxes for a replace operation are:

REPLACE document IN collection options
REPLACE keyExpression WITH document IN collection options

collection must contain the name of the collection in which the documents should be replaced. document is the replacement document. When using the first syntax, document must also contain the _key attribute to identify the document to be replaced.

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

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

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

When using the second syntax, keyExpression provides the document identification. This can either be a string (which must then contain the document key) or a document, which must contain a _key attribute.

The following queries are equivalent:

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

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

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

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

A replace will fully replace an existing document, but it will not modify the values of internal attributes (such as _id, _key, _from and _to). Replacing a document will modify a document's revision number with a server-generated value.

A replace operation may update arbitrary documents which do not need to be identical to the ones produced by a preceding FOR statement:

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

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

Setting query options

options can be used 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 { _key: CONCAT('test', i) } WITH { foobar: true } IN users OPTIONS { ignoreErrors: true }

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

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

In order to not accidentially overwrite documents that have been updated 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 }

In contrast to the MMFiles engine, 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 conficts 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._key 
  WITH { replaced: true } IN collection 
  OPTIONS { exclusive: true }

Returning the modified documents

The modified documents can also be returned by the query. In this case, the REPLACE statement must be followed by a RETURN statement (intermediate LET statements are allowed, too). The OLD pseudo-value can be used to refer to document revisions before the replace, and NEW refers to document revisions after the replace.

Both OLD and NEW will 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 will be 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')