REMOVE

The REMOVE keyword can be used to remove documents from a collection. On a single server, the document removal 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 remove operation may not be transactional, especially if it involves different shards and/or database servers.

Each REMOVE operation is restricted to a single collection, and the collection name must not be dynamic. Only a single REMOVE 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 syntax for a remove operation is:

REMOVE keyExpression IN collection options

collection must contain the name of the collection to remove the documents from. keyExpression must be an expression that contains 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 thus equivalent:

FOR u IN users
  REMOVE { _key: u._key } IN users

FOR u IN users
  REMOVE u._key IN users

FOR u IN users
  REMOVE u IN users

Note: A remove operation can remove arbitrary documents, and the documents do not need to be identical to the ones produced by a preceding FOR statement:

FOR i IN 1..1000
  REMOVE { _key: CONCAT('test', i) } IN users

FOR u IN users
  FILTER u.active == false
  REMOVE { _key: u._key } IN backup

A single document can be removed as well, using a document key string or a document with _key attribute:

REMOVE 'john' IN users

LET doc = DOCUMENT('users/john')
REMOVE doc IN users

The restriction of a single remove operation per query and collection applies. The following query causes an access after data-modification error because of the third remove operation:

REMOVE 'john' IN users
REMOVE 'john' IN backups // OK, different collection
REMOVE 'mary' IN users   // Error, users collection again

Setting query options

options can be used to suppress query errors that may occur when trying to remove non-existing documents. For example, the following query will fail if one of the to-be-deleted documents does not exist:

FOR i IN 1..1000
  REMOVE { _key: CONCAT('test', i) } IN users

By specifying the ignoreErrors query option, these errors can be suppressed so the query completes:

FOR i IN 1..1000
  REMOVE { _key: CONCAT('test', i) } IN users OPTIONS { ignoreErrors: true }

To make sure data has been written to disk when a query returns, there is the waitForSync query option:

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

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

FOR i IN 1..1000
  REMOVE { _key: CONCAT('test', i), _rev: "1287623" } 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 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._key 
      WITH { replaced: true } 
      OPTIONS { exclusive: true }

Returning the removed documents

The removed documents can also be returned by the query. In this case, the REMOVE statement must be followed by a RETURN statement (intermediate LET statements are allowed, too).REMOVE introduces the pseudo-value OLD to refer to the removed documents:

REMOVE keyExpression IN collection options RETURN OLD

Following is an example using a variable named removed for capturing the removed documents. For each removed document, the document key will be returned.

FOR u IN users
  REMOVE u IN users 
  LET removed = OLD 
  RETURN removed._key