A thorough discussion of Elgg’s data model design and motivation.


In Elgg, everything runs on a unified data model based on atomic units of data called entities.

Plugins are discouraged from interacting directly with the database, which creates a more stable system and a better user experience becase content created by different plugins can be mixed together in consistent ways. With this approach, plugins are faster to develop, and are at the same time much more powerful.

Every entity in the system inherits the ElggEntity class. This class controls access permissions, ownership

You can extend entities with extra information in two ways:

Metadata: This is information describing the entity, usually
added by the author of the entity when the entity is created. For example, tags, an ISBN number, a file location, or source language is metadata.
Annotations: This is information about the entity, usually
added by a third party after the entity is created. For example, ratings, likes, and votes are annotations. (Comments were before 1.9.)


The Elgg data model diagram

The Elgg data model diagram


ElggEntity is the base class for the Elgg data model.

Users, Objects, Groups, Sites

ElggEntity has four main specializations, which provide extra properties and methods to more easily handle different kinds of data.

ElggObject: content like blog posts, uploaded files and bookmarks ElggUser: a system user ElggSite: each Elgg site within an Elgg installation ElggGroup: multi-user collaborative systems (called “Communities” in prior versions of Elgg)

The benefit of such an approach is that, apart from modelling data with greater ease, a common set of functions is available to handle objects, regardless of their (sub)type.

Each of these have their own properties that they bring to the table: ElggObjects have a title and description, ElggUsers have a username and password, and so on. However, because they all inherit ElggEntity, they each have a number of core properties and behaviours in common.

  • A numeric Globally Unique IDentifier (See GUIDs).
  • Access permissions. (When a plugin requests data, it never gets to touch data that the current user doesn’t have permission to see.)
  • An arbitrary subtype. For example, a blog post is an ElggObject with a subtype of “blog”. Subtypes aren’t predefined; they can be any unique way to describe a particular kind of entity. “blog”, “forum”, “foo”, “bar”, “loafofbread” and “pyjamas” are all valid subtypes.
  • An owner.
  • The site that the entity belongs to.
  • A container, usually used to associate a group’s content with the group.


A GUID is an integer that uniquely identifies every entity in an Elgg installation (a Globally Unique IDentifier). It’s assigned automatically when the entity is first saved and can never be changed.

Some Elgg API functions work with GUIDs instead of ElggEntity objects.


The ElggObject entity type represents arbitrary content within an Elgg install; things like blog posts, uploaded files, etc.

Beyond the standard ElggEntity properties, ElggObjects also support:

  • title The title of the object (HTML escaped text)
  • description A description of the object (HTML)

Most other data about the object is generally stored via metadata.


The ElggUser entity type represents users within an Elgg install. These will be set to disabled until their accounts have been activated (unless they were created from within the admin panel).

Beyond the standard ElggEntity properties, ElggUsers also support:

  • name The user’s plain text name. e.g. “Hugh Jackman”
  • username Their login name. E.g. “hjackman”
  • password A hashed version of their password
  • salt The salt that their password has been hashed with
  • email Their email address
  • language Their default language code.
  • code Their session code (moved to a separate table in 1.9).
  • last_action The UNIX timestamp of the last time they loaded a page
  • prev_last_action The previous value of last_action
  • last_login The UNIX timestamp of their last log in
  • prev_last_login the previous value of last_login


The ElggSite entity type represents sites within your Elgg install. Most installs will have only one.

Beyond the standard ElggEntity properties, ElggSites also support:

  • name The site name
  • description A description of the site
  • url The address of the site


The ElggGroup entity type represents an association of Elgg users. Users can join, leave, and post content to groups.

Beyond the standard ElggEntity properties, ElggGroups also support:

  • name The group’s name (HTML escaped text)
  • description A description of the group (HTML)

ElggGroup has addition methods to manage content and membership.

The Groups plugin

Not to be confused with the entity type ElggGroup, Elgg comes with a plugin called “Groups” that provides a default UI/UX for site users to interact with groups. Each group is given a discussion forum and a profile page linking users to content within the group.

You can alter the user experience via the traditional means of extending plugins or completely replace the Groups plugin with your own.

Because ElggGroup can be subtyped like all other ElggEntities, you can have multiple types of groups running on the same site.

Writing a group-aware plugin

Plugin owners need not worry too much about writing group-aware functionality, but there are a few key points:

Adding content

By passing along the group as container_guid via a hidden input field, you can use a single form and action to add both user and group content.

Use can_write_to_container to determine whether or not the current user has the right to add content to a group.

Be aware that you will then need to pass the container GUID or username to the page responsible for posting and the accompanying value, so that this can then be stored in your form as a hidden input field, for easy passing to your actions. Within a “create” action, you’ll need to take in this input field and save it as a property of your new element (defaulting to the current user’s container):

$user = elgg_get_logged_in_user_entity();
$container_guid = (int)get_input('container_guid');
if ($container_guid) {
    if (!can_write_to_container($user->guid, $container_guid)) {
        // register error and forward
} else {
    $container_guid = elgg_get_logged_in_user_guid();

$object = new ElggObject;
$object->container_guid = $container_guid;


$container = get_entity($container_guid);

Usernames and page ownership

Groups have a simulated username of the form group:GUID, which you can get the value of by checking $group->username. If you pass this username to a page on the URL line as part of the username variable (i.e., /yourpage?username=group:nnn), Elgg will automatically register that group as being the owner of the page (unless overridden).

Juggling users and groups

In fact, [[Engine/DataModel/Entities/ElggGroup|ElggGroup]] simulates most of the methods of [[Engine/DataModel/Entities/ElggUser|ElggUser]]. You can grab the icon, name etc using the same calls, and if you ask for a group’s friends, you’ll get its members. This has been designed specifically for you to alternate between groups and users in your code easily.


Entities have a owner_guid GUID property, which defines its owner. Typically this refers to the GUID of a user, although sites and users themselves often have no owner (a value of 0).

The ownership of an entity dictates, in part, whether or not you can access or edit that entity.


In order to easily search content by group or by user, content is generally set to be “contained” by either the user who posted it, or the group to which the user posted. This means the new object’s container_guid property will be set to the GUID of the current ElggUser or the target ElggGroup.

E.g., three blog posts may be owned by different authors, but all be contained by the group they were posted to.

Note: This is not always true. Comment entities are contained by the object commented upon, and in some 3rd party plugins the container may be used to model a parent-child relationship between entities (e.g. a “folder” object containing a file object).


Annotations are pieces of data attached to an entity that allow users to leave ratings, or other relevant feedback. A poll plugin might register votes as annotations. Before Elgg 1.9, comments and group discussion replies were stored as annotations.

Annotations are stored as instances of the ElggAnnotation class.

Each annotation has:

  • An internal annotation type (like comment)
  • A value (which can be a string or integer)
  • An access permission distinct from the entity it’s attached to
  • An owner

Adding an annotation

The easiest way to annotate is to use the annotate method on an entity, which is defined as:

public function annotate(
    $name,           // The name of the annotation type (eg 'comment')
    $value,          // The value of the annotation
    $access_id = 0,  // The access level of the annotation
    $owner_id = 0,   // The annotation owner, defaults to current user
    $vartype = ""    // 'text' or 'integer'

For example, to leave a rating on an entity, you might call:

$entity->annotate('rating', $rating_value, $entity->access_id);

Reading annotations

To retrieve annotations on an object, you can call the following method:

$annotations = $entity->getAnnotations(
    $name,    // The type of annotation
    $limit,   // The number to return
    $offset,  // Any indexing offset
    $order,   // 'asc' or 'desc' (default 'asc')

If your annotation type largely deals with integer values, a couple of useful mathematical functions are provided:

$averagevalue = $entity->getAnnotationsAvg($name);  // Get the average value
$total = $entity->getAnnotationsSum($name);         // Get the total value
$minvalue = $entity->getAnnotationsMin($name);      // Get the minimum value
$maxvalue = $entity->getAnnotationsMax($name);      // Get the maximum value

Useful helper functions


If you want to provide comment functionality on your plugin objects, the following function will provide the full listing, form and actions:

function elgg_view_comments(ElggEntity $entity)


Metadata in Elgg allows you to store extra data on an entity beyond the built-in fields that entity supports. For example, ElggObjects only support the basic entity fields plus title and description, but you might want to include tags or an ISBN number. Similarly, you might want users to be able to save a date of birth.

Under the hood, metadata is stored as an instance of the ElggMetadata class, but you don’t need to worry about that in practice (although if you’re interested, see the ElggMetadata class reference). What you need to know is:

  • Metadata has an owner and access ID, both of which may be different to the owner of the entity it’s attached to
  • You can potentially have multiple items of each type of metadata attached to a single entity

The simple case

Adding metadata

To add a piece of metadata to an entity, just call:

$entity->metadata_name = $metadata_value;

For example, to add a date of birth to a user:

$user->dob = $dob_timestamp;

Or to add a couple of tags to an object:

$object->tags = array('tag one', 'tag two', 'tag three');

When adding metadata like this:

  • The owner is set to the currently logged-in user
  • Access permissions are inherited from the entity
  • Reassigning a piece of metadata will overwrite the old value

This is suitable for most purposes. Be careful to note which attributes are metadata and which are built in to the entity type that you are working with. You do not need to save an entity after adding or updating metadata. You do need to save an entity if you have changed one of its built in attributes. As an example, if you changed the access id of an ElggObject, you need to save it or the change isn’t pushed to the database.

Reading metadata

To retrieve metadata, treat it as a property of the entity:

$tags_value = $object->tags;

Note that this will return the absolute value of the metadata. To get metadata as an ElggMetadata object, you will need to use the methods described in the finer control section below.

If you stored multiple values in this piece of metadata (as in the “tags” example above), you will get an array of all those values back. If you stored only one value, you will get a string or integer back. Storing an array with only one value will return a string back to you. E.g.

$object->tags = array('tag');
$tags = $object->tags;
// $tags will be the string "tag", NOT array('tag')

To always get an array back, simply cast to an array;

$tags = (array)$object->tags;

Finer control

Adding metadata

If you need more control, for example to assign an access ID other than the default, you can use the create_metadata function, which is defined as follows:

function create_metadata(
    $entity_guid,           // The GUID of the parent entity
    $name,                  // The name of the metadata (eg 'tags')
    $value,                 // The metadata value
    $value_type,            // Currently either 'string' or 'integer'
    $owner_guid,            // The owner of the metadata
    $access_id = 0,         // The access restriction
    $allow_multiple = false // Do we have more than one value?

For single values, you can therefore write metadata as follows (taking the example of a date of birth attached to a user):

create_metadata($user_guid, 'dob', $dob_timestamp, 'integer', $_SESSION['guid'], $access_id);

For multiple values, you will need to iterate through and call create_metadata on each one. The following piece of code comes from the profile save action:

$i = 0;
foreach ($value as $interval) {
    $multiple = ($i != 1);
    create_metadata($user->guid, $shortname, $interval, 'text', $user->guid, $access_id, $multiple);

Note that the allow multiple setting is set to false in the first iteration and true thereafter.

Reading metadata

elgg_get_metadata is the best function for retrieving metadata as ElggMetadata objects:

E.g., to retrieve a user’s DOB

    'metadata_name' => 'dob',
    'metadata_owner_guid' => $user_guid,

Or to get all metadata objects:

    'metadata_owner_guid' => $user_guid,
    'limit' => 0,

Common mistakes

“Appending” metadata

Note that you cannot “append” values to metadata arrays as if they were normal php arrays. For example, the following will not do what it looks like it should do.

$object->tags[] = "tag four";

Trying to store hashmaps

Elgg does not support storing ordered maps (name/value pairs) in metadata. For example, the following does not work as you might first expect it to:

// Won't work!! Only the array values are stored
$object->tags = array('one' => 'a', 'two' => 'b', 'three' => 'c');

You can instead store the information like so:

$object->one = 'a';
$object->two = 'b';
$object->three = 'c';

Storing GUIDs in metadata

Though there are some cases to store entity GUIDs in metadata, Relationships are a much better construct for relating entities to each other.


Relationships allow you to bind entities together. Examples: an artist has fans, a user is a member of an organization, etc.

The class ElggRelationship models a directed relationship between two entities, making the statement:

{subject} is a {noun} of {target}.”
API name Models Represents
guid_one The subject Which entity is being bound
relationship The noun The type of relationship
guid_two The target The entity to which the subject is bound

The type of relationship may alternately be a verb, making the statement:

{subject} {verb} {target}.”

E.g. User A “likes” blog post B

Each relationship has direction. Imagine an archer shoots an arrow at a target; The arrow moves in one direction, binding the subject (the archer) to the target.

A relationship does not imply reciprocity. A follows B does not imply that B follows A.

Relationships_ do not have access control. They’re never hidden from view and can be edited with code at any privilege level, with the caveat that the entities in a relationship may be invisible due to access control!

Working with relationships

Creating a relationship

E.g. to establish that “$user is a fan of $artist” (user is the subject, artist is the target):

// option 1
$success = add_entity_relationship($user->guid, 'fan', $artist->guid);

// option 2
$success = $user->addRelationship($artist->guid, 'fan');

This triggers the event [create, relationship], passing in the created ElggRelationship object. If a handler returns false, the relationship will not be created and $success will be false.

Verifying a relationship

E.g. to verify that “$user is a fan of $artist”:

if (check_entity_relationship($user->guid, 'fan', $artist->guid)) {
    // relationship exists

Note that, if the relationship exists, check_entity_relationship() returns an ElggRelationship object:

$relationship = check_entity_relationship($user->guid, 'fan', $artist->guid);
if ($relationship) {
    // use $relationship->id or $relationship->time_created

Deleting a relationship

E.g. to be able to assert that “$user is no longer a fan of $artist”:

$was_removed = remove_entity_relationship($user->guid, 'fan', $artist->guid);

This triggers the event [delete, relationship], passing in the associated ElggRelationship object. If a handler returns false, the relationship will remain, and $was_removed will be false.

Other useful functions:

  • delete_relationship() : delete by ID
  • remove_entity_relationships() : delete those relating to an entity (note: in versions before Elgg 1.9, this did not trigger delete events)

Access Control

Granular access controls are one of the fundamental design principles in Elgg, and a feature that has been at the centre of the system throughout its development. The idea is simple: a user should have full control over who sees an item of data he or she creates.

Access controls in the data model

In order to achieve this, every entity, annotation and piece of metadata contains an access_id property, which in turn corresponds to one of the pre-defined access controls or an entry in the access_collections database table.

Pre-defined access controls

  • 0 Private.
  • 1 Logged in users.
  • 2 Public data.

User defined access controls

You may define additional access groups and assign them to an entity, annotation or metadata. A number of functions have been defined to assist you; see the access library reference for more information.

How access affects data retrieval

All data retrieval functions above the database layer - for example get_entities and its cousins - will only return items that the current user has access to see. It is not possible to retrieve items that the current user does not have access to. This makes it very hard to create a security hole for retrieval.

Write access

The following rules govern write access:

  • The owner of an entity can always edit it
  • The owner of a container can edit anything therein (note that this does not mean that the owner of a group can edit anything therein)
  • Admins can edit anything

You can override this behaviour using a plugin hook called permissions_check, which passes the entity in question to any function that has announced it wants to be referenced. Returning true will allow write access; returning false will deny it. See the plugin hook reference for permissions_check for more details.


The database contains a number of primary tables and secondary tables. Its schema table is stored in /engine/schema/mysql.sql.

Each table is prefixed by “prefix_”, this is replaced by the Elgg framework during installation.

Main tables

This is a description of the main tables. Keep in mind that in a given Elgg installation, the tables will have a prefix (typically “elgg_”).

Table: entities

This is the main Entities table containing Elgg users, sites, objects and groups. When you first install Elgg this is automatically populated with your first site.

It contains the following fields:

  • guid An auto-incrementing counter producing a GUID that uniquely identifies this entity in the system.
  • type The type of entity - object, user, group or site
  • subtype A link to the entity_subtypes table.
  • owner_guid The GUID of the owner’s entity.
  • site_guid The site the entity belongs to.
  • container_guid The GUID this entity is contained by - either a user or a group.
  • access_id Access controls on this entity.
  • time_created Unix timestamp of when the entity is created.
  • time_updated Unix timestamp of when the entity was updated.
  • enabled If this is ‘yes’ an entity is accessible, if ‘no’ the entity has been disabled (Elgg treats it as if it were deleted without actually removing it from the database).

Table: entity_subtypes

This table contains entity subtype information:

  • id A counter.
  • type The type of entity - object, user, group or site.
  • subtype The subtype name as a string.
  • class Optional class name if this subtype is linked with a class

Table: metadata

This table contains Metadata, extra information attached to an entity.

  • id A counter.
  • entity_guid The entity this is attached to.
  • name_id A link to the metastrings table defining the name table.
  • value_id A link to the metastrings table defining the value.
  • value_type The value class, either text or an integer.
  • owner_guid The owner GUID of the owner who set this item of metadata.
  • access_id An Access controls on this item of metadata.
  • time_created Unix timestamp of when the metadata is created.
  • enabled If this is ‘yes’ an item is accessible, if ‘no’ the item has been deleted.

Table: annotations

This table contains Annotations, this is distinct from Metadata.

  • id A counter.
  • entity_guid The entity this is attached to.
  • name_id A link to the metastrings table defining the type of annotation.
  • value_id A link to the metastrings table defining the value.
  • value_type The value class, either text or an integer.
  • owner_guid The owner GUID of the owner who set this item of metadata.
  • access_id An Access controls on this item of metadata.
  • time_created Unix timestamp of when the metadata is created.
  • enabled If this is ‘yes’ an item is accessible, if ‘no’ the item has been deleted.

Table: relationships

This table defines Relationships, these link one entity with another.

  • guid_one The GUID of the subject entity.
  • relationship The type of the relationship.
  • guid_two The GUID of the target entity.

Table: objects_entity

Extra information specifically relating to objects. These are split in order to reduce load on the metadata table and make an obvious difference between attributes and metadata.

Table: sites_entity

Extra information specifically relating to sites. These are split in order to reduce load on the metadata table and make an obvious difference between attributes and metadata.

Table: users_entity

Extra information specifically relating to users. These are split in order to reduce load on the metadata table and make an obvious difference between attributes and metadata.

Table: groups_entity

Extra information specifically relating to groups. These are split in order to reduce load on the metadata table and make an obvious difference between attributes and metadata.

Table: metastrings

Metastrings contain the actual string of metadata which is linked to by the metadata and annotations tables.

This is to avoid duplicating strings, saving space and making database lookups more efficient.

Core developers will place schema upgrades in /engine/schema/upgrades/*.