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sed -n '<from>,<to>p' SKILL.md one section rather than reading the file; ./mxcli syntax <topic> answers how do I write this for a fraction of either.

lines	section
32-49	Access to System entities has a ceiling
50-56	Reference files
57-66	When to Use This Skill
67-127	How to Reference System Entities
128-220	1. User Management & Authentication
221-274	2. File Management
275-332	3. HTTP / Web Services
333-359	4. Error Handling
360-383	7. Utility Entities
384-448	8. Enumerations
449-480	9. Inheritance Hierarchies
481-536	10. Common Patterns
537-539	Source

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Mendix System Module Reference

The System module is a built-in Mendix module present in every application. It provides core entities for user management, file handling, workflows, task queues, HTTP services, and more. These entities are not defined in the application's domain model but are available for use in microflows, pages, associations, and Java actions.

Access to System entities has a ceiling

No project module can widen access to System.User, System.Workflow or System.WorkflowUserTask. Their access comes from the System module's own roles, and a grant in your module cannot raise it — so any UI over them is Administrator-only unless the data is denormalised into entities your own module owns.

A microflow data source is not the way out. It moves the ROWS, not the MEMBERS: the retrieve is unconstrained, but the runtime re-applies entity access when it serializes those objects to the client, so the list is the right length and every field is blank (measured on 11.14.0, ako/mxcli#587). Read the members inside the microflow and return an object your module owns.

It fails silently: a combo box over System.User lists the current user only, a grid over System.Workflow renders empty, and mx check, mxcli lint and mxcli report all pass. See the System-module ceiling section in manage-security for the measurement and the remedies.

Reference files

• reference/workflow-and-queues.md — the workflow engine's entities (WorkflowInstance, UserTask, the state enumerations and how they relate), plus task queues and scheduled events. Open it when querying workflow state or wiring anything to a queue.

When to Use This Skill

Use this skill when:

• Creating associations to System entities (e.g., linking a record to System.User)
• Working with file uploads/downloads (System.FileDocument, System.Image)
• Implementing workflow-related features
• Writing microflows that reference System types
• Understanding what's available out of the box in Mendix
• Designing security models that reference System.User and System.UserRole

How to Reference System Entities

In MDL, reference System entities with the System. prefix:

mdl 1;
-- Association to the current user. The direction is not a style choice: YOUR
-- entity must be the FROM side (see "The FROM entity must be yours" below).
create association MyModule.Order_CreatedBy
from MyModule.Order to System.User
type reference;

-- Entity that generalizes FileDocument for file uploads
create persistent entity MyModule.Invoice extends System.FileDocument (
  InvoiceNumber: string(50),
  IssueDate: datetime
);

-- Entity that generalizes Image for image uploads
create persistent entity MyModule.ProductPhoto extends System.Image (
  PhotoCaption: string(200),
  IsPrimary: boolean default false
);

The FROM entity must be yours

An association is stored in the module of its FROM entity — Mendix has no other place to put it. So from System.User to MyModule.Order is not a differently-shaped association; it is one Mendix cannot store. Written anyway, its ParentPointer names an element in the System unit and the project stops opening:

KeyNotFoundException: The given key '4a25f08b-…' was not present in the dictionary
  at StreamingBsonUnitReader.ResolvePostponedProperties()

That is a load failure, not a build error, so Studio Pro cannot open it either and the stack trace names no document. mxcli refuses it as MDL070 at check time, before anything is written.

This is not about System. Any remote FROM module does the same thing — create association MyModule.X from Administration.Account to MyModule.Y fails identically. The rule is the module boundary.

Three ways out, in the order to consider them:

Situation	Do this
The FK genuinely belongs on your entity	Reverse it: from MyModule.Order to System.User
Many-to-many across the boundary	A join entity in your module with a reference to each side
The association really belongs to the other module	Declare it there — but never in System or a Marketplace module, which must not be written to

The join entity is usually the better model anyway: a row that says "this user plans this department" is something you can hang attributes and XPath constraints on, which a bare reference set is not.

---

1. User Management & Authentication

System.User

The central user entity. All application users are instances of System.User or a specialization (e.g., Administration.Account).

Attribute	Type	Description
Name	String	Username (login name)
Password	String	Hashed password (write-only)
LastLogin	DateTime	Timestamp of last successful login
Blocked	Boolean	Whether user account is blocked
BlockedSince	DateTime	When the account was blocked
Active	Boolean	Whether the account is active
FailedLogins	Integer	Count of consecutive failed login attempts
WebServiceUser	Boolean	Whether this is a web service account
IsAnonymous	Boolean	Whether this is an anonymous user

Association	Target	Type	Description
UserRoles	System.UserRole	Many-to-Many	Roles assigned to this user. In XPath it is System.UserRoles — not System.User_UserRoles, which fails the build with CE1613 "The selected association … no longer exists"
User_Language	System.Language	Many-to-One	User's preferred language
User_TimeZone	System.TimeZone	Many-to-One	User's timezone

Common usage patterns:

• Associate application entities to System.User for audit trails (CreatedBy, ModifiedBy)
• Specialize System.User (via Administration.Account) to add custom profile attributes
• Use [%CurrentUser%] token in XPath to filter by logged-in user
• Check $user/Active and $user/Blocked for access control

System.UserRole

Represents an application user role (e.g., Administrator, User, Manager).

Attribute	Type	Description
ModelGUID	String	GUID from the model definition
Name	String	Role name as defined in the model
Description	String	Role description

Association	Target	Type	Description
UserRole_GrantableRoles	System.UserRole	Many-to-Many	Roles that holders of this role can assign to others

System.Session

Active user sessions.

Attribute	Type	Description
SessionId	String	Unique session identifier
CSRFToken	String	Cross-site request forgery token
LastActive	DateTime	Last activity timestamp

Association	Target	Type	Description
Session_User	System.User	Many-to-One	User who owns this session

System.Language

Available languages for internationalization.

Attribute	Type	Description
Code	String	Language code (e.g., en_US, nl_NL)
Description	String	Human-readable language name

System.TimeZone

Available time zones.

Attribute	Type	Description
Code	String	Timezone identifier (e.g., Europe/Amsterdam)
Description	String	Human-readable timezone name
RawOffset	Integer	Offset from UTC in milliseconds

System.TokenInformation

Authentication tokens (e.g., for "remember me" or API tokens).

Attribute	Type	Description
Token	String	Hashed token value (write-only)
ExpiryDate	DateTime	When the token expires
UserAgent	String	Browser/client user agent

Association	Target	Type	Description
TokenInformation_User	System.User	Many-to-One	User who owns this token

---

2. File Management

System.FileDocument

Base entity for all file storage. Specialize this entity to create custom file types.

Attribute	Type	Description
FileID	Long	Internal file identifier
Name	String	File name (including extension)
DeleteAfterDownload	Boolean	Auto-delete after first download
Contents	Binary	The file binary content
HasContents	Boolean	Whether file content has been uploaded
Size	Long	File size in bytes

Usage: Create a specialization to store typed files:

mdl 1;
create persistent entity MyModule.Attachment extends System.FileDocument (
  description: string(500),
  Category: MyModule.AttachmentCategory
);

create association MyModule.Order_Attachments
from MyModule.Order to MyModule.Attachment
type ReferenceSet;

System.Image

Extends System.FileDocument with image-specific features.

Attribute	Type	Description
PublicThumbnailPath	String	Path to auto-generated thumbnail
EnableCaching	Boolean	Whether the browser should cache this image

Usage: Specialize for application images:

mdl 1;
create persistent entity MyModule.ProductPhoto extends System.Image (
  PhotoCaption: string(200),
  SortOrder: integer default 0
);

System.SynchronizationErrorFile

File attachment for offline synchronization errors. Extends System.FileDocument.

---

3. HTTP / Web Services

System.HttpMessage (base, non-persistent)

Base entity for HTTP messages. Not stored in the database.

Attribute	Type	Description
HttpVersion	String	HTTP version (e.g., 1.1)
Content	String	Message body content

System.HttpRequest (extends HttpMessage, non-persistent)

Attribute	Type	Description
Uri	String	Request URI

System.HttpResponse (extends HttpMessage, non-persistent)

Attribute	Type	Description
StatusCode	Integer	HTTP status code (200, 404, 500, etc.)
ReasonPhrase	String	Status reason phrase

System.HttpHeader (non-persistent)

Attribute	Type	Description
Key	String	Header name
Value	String	Header value

Association	Target	Type	Description
HttpHeaders	System.HttpMessage	Many-to-One	Parent HTTP message

Usage: These entities are used in published/consumed REST services and Java actions that handle HTTP requests and responses directly.

System.ConsumedODataConfiguration

Configuration for consumed OData services.

Attribute	Type	Description
ServiceUrl	String	OData service endpoint URL
ProxyConfiguration	Enum (ProxyConfiguration)	Proxy setting: UseAppSettings, Override, NoProxy
ProxyHost	String	Proxy hostname (when Override)
ProxyPort	Integer	Proxy port (when Override)
ProxyUsername	String	Proxy authentication username
ProxyPassword	String	Proxy authentication password

System.ODataResponse

Attribute	Type	Description
Count	Long	Total record count from OData response

---

4. Error Handling

System.Error (non-persistent)

Attribute	Type	Description
ErrorType	String	Error category/type
Message	String	Error message
Stacktrace	String	Full stack trace

System.SoapFault (extends Error, non-persistent)

SOAP-specific fault information. Extends System.Error with SOAP fault details.

System.SynchronizationError

Tracks offline mobile synchronization failures.

Attribute	Type	Description
Reason	String	Why synchronization failed
ObjectId	String	ID of the object that failed
ObjectType	String	Entity type of the failed object
ObjectContent	String	Serialized state of the object

---

7. Utility Entities

System.Paging (non-persistent)

Paging information for data retrieval in custom Java actions.

Attribute	Type	Description
PageNumber	Long	Current page number
IsSortable	Boolean	Whether sorting is supported
SortAttribute	String	Attribute to sort by
SortAscending	Boolean	Sort direction
HasMoreData	Boolean	Whether more pages exist

System.UserReportInfo

Information for user management reports (internal use).

System.ProxyConfiguration

HTTP proxy settings (internal use).

---

8. Enumerations

Inspect any of these from the CLI rather than copying from here — the values are case-sensitive, and a wrong one is only caught at build time as CE1613 "The selected enumeration value no longer exists":

mxcli -p app.mpr describe enumeration System.WorkflowActivityType
mxcli -p app.mpr list enumerations            # includes the System module

Two of these names are easy to confuse, and mixing them up is the mistake that CE1613 usually reports: WorkflowActivityState has Finished, while WorkflowActivityExecutionState has Completed. They are different enumerations on different entities.

System enumerations are read-only — they are built into the platform, not stored in the project, so create/alter/drop/move enumeration System.… is refused. describe prints them as -- comment lines for that reason.

ContextType

System, User, Anonymous, ScheduledEvent

DeviceType

Phone, Tablet, Desktop

EventStatus

Running, Completed, Error, Stopped

ProxyConfiguration

UseAppSettings, Override, NoProxy

QueueTaskStatus

Idle, Running, Completed, Failed, Retrying, Aborted, Incompatible

UnreferencedFileState

New, Obsolete, Deleted

UserType

Internal, External

WorkflowActivityExecutionState

Created, InProgress, Completed, Paused, Aborted, Failed

WorkflowActivityState

Started, Suspended, Finished, Replaced, Aborted, Failed

WorkflowActivityType

Start, End, ExclusiveSplit, ParallelSplit, ParallelSplitBranchStopper, ParallelSplitMerge, UserTask, CallMicroflow, CallWorkflow, JumpTo, MultiInputUserTask, WaitForNotification, WaitForTimer, EndOfBoundaryEventPath, NonInterruptingTimerEvent, InterruptingTimerEvent

WorkflowCurrentActivityAction

DoNothing, JumpTo

WorkflowEventType

WorkflowCompleted, WorkflowInitiated, WorkflowRestarted, WorkflowFailed, WorkflowAborted, WorkflowPaused, WorkflowUnpaused, WorkflowRetried, WorkflowUpdated, WorkflowUpgraded, WorkflowConflicted, WorkflowResolved, WorkflowJumpToOptionApplied, StartEventExecuted, EndEventExecuted, DecisionExecuted, JumpExecuted, ParallelSplitExecuted, ParallelMergeExecuted, CallWorkflowStarted, CallWorkflowEnded, CallMicroflowStarted, CallMicroflowEnded, WaitForNotificationStarted, WaitForNotificationEnded, WaitForTimerStarted, WaitForTimerEnded, UserTaskStarted, MultiUserTaskOutcomeSelected, UserTaskEnded, NonInterruptingTimerEventExecuted, InterruptingTimerEventExecuted

WorkflowState

InProgress, Paused, Completed, Aborted, Incompatible, Failed

WorkflowUserTaskCompletionType

Single, Veto, Consensus, Majority, Threshold, Microflow

WorkflowUserTaskState

Created, InProgress, Completed, Paused, Aborted, Failed

9. Inheritance Hierarchies

System.User
  └── Administration.Account (adds FullName, Email, etc.)

System.FileDocument
  ├── System.Image
  └── System.SynchronizationErrorFile

System.HttpMessage (non-persistent)
  ├── System.HttpRequest
  └── System.HttpResponse

System.Error (non-persistent)
  └── System.SoapFault

Key point for MDL: When creating entities that store files or images, use extends:

mdl 1;
create persistent entity MyModule.Document extends System.FileDocument (
  title: string(200),
  version: integer default 1
);

create persistent entity MyModule.Photo extends System.Image (
  AltText: string(200)
);

10. Common Patterns

Audit Trail (CreatedBy / ModifiedBy)

mdl 1;
create association MyModule.Order_CreatedBy
from MyModule.Order to System.User
type reference;

create association MyModule.Order_ModifiedBy
from MyModule.Order to System.User
type reference;

File Attachments

mdl 1;
create persistent entity MyModule.Attachment extends System.FileDocument (
  description: string(500)
);

create association MyModule.Order_Attachments
from MyModule.Order to MyModule.Attachment
type ReferenceSet;

Workflow Context Object

mdl 1;
-- Application entity that serves as workflow context
create persistent entity MyModule.ExpenseReport (
  Amount: decimal,
  description: string(500),
  status: MyModule.ApprovalStatus default 'Draft'
);

-- Associate with workflow instance
create association MyModule.ExpenseReport_Workflow
from MyModule.ExpenseReport to System.Workflow
type reference;

Task Inbox Page

User tasks can be displayed in pages by retrieving System.WorkflowUserTask where the current user is in the target users or assignees.

XPath Tokens for System Entities

[%CurrentUser%]     -- The logged-in System.User
[%CurrentObject%]   -- The current context object

Source

This reference was extracted from the Java proxy files in javasource/system/proxies/ of a Mendix 10.x application. The System module domain model is not exposed in MPR/BSON files but is defined internally by the Mendix runtime. Proxy files serve as the definitive reference for available entities, attributes, and associations.