MPR File Format

Mendix projects are stored in .mpr files, which are SQLite databases containing BSON-encoded model elements. This page provides an overview of the format; see v1 vs v2 for version-specific details.

Structure

An MPR file is a standard SQLite database with two key tables:

Unit Table

The Unit table stores document metadata:

Column	Description
`UnitID`	Binary UUID identifying the document
`ContainerID`	Parent module UUID
`ContainmentName`	Relationship name (e.g., `documents`)
`UnitType`	Fully qualified type name
`Name`	Document name

UnitContents Table (v1 only)

In MPR v1, the UnitContents table stores the actual BSON document content:

Column	Description
`UnitID`	Binary UUID matching the Unit table
`Contents`	BSON blob containing the full document

In MPR v2, document contents are stored as individual files in the mprcontents/ folder instead.

Unit Types

Every document in a Mendix project has a unit type:

UnitType	Document Type
`DomainModels$DomainModel`	Domain model (entities, associations)
`DomainModels$ViewEntitySourceDocument`	OQL query for VIEW entities
`Microflows$Microflow`	Microflow definition
`Microflows$Nanoflow`	Nanoflow definition
`Pages$Page`	Page definition
`Pages$Layout`	Layout definition
`Pages$Snippet`	Snippet definition
`Pages$BuildingBlock`	Building block definition
`Enumerations$Enumeration`	Enumeration definition
`JavaActions$JavaAction`	Java action definition
`Security$ProjectSecurity`	Project security settings
`Security$ModuleSecurity`	Module security settings
`Navigation$NavigationDocument`	Navigation profile
`Settings$ProjectSettings`	Project settings
`BusinessEvents$BusinessEventService`	Business event service

BSON Document Structure

Every BSON document contains at minimum:

Field	Type	Description
`$ID`	Binary (UUID)	Unique identifier for this element
`$Type`	String	Fully qualified type name (using storageName, not qualifiedName)

ID Format

IDs are stored as BSON Binary subtype 0 (generic) containing UUID bytes:

{
  "$ID": {
    "Subtype": 0,
    "Data": "base64-encoded-uuid"
  }
}

Array Convention

Arrays in Mendix BSON have an integer count as the first element:

{
  "Attributes": [
    3,              // Count/type prefix
    { ... },        // First attribute
    { ... }         // Second attribute
  ]
}

The prefix value (typically 2 or 3) indicates the array type. When writing arrays, you must include this prefix. When parsing, skip the first element.

Reference Types

Reference Type	Storage Format	Example
`BY_ID_REFERENCE`	Binary UUID	Index `AttributePointer`
`BY_NAME_REFERENCE`	Qualified name string	ValidationRule `Attribute`
`PART`	Embedded BSON object	Child objects serialized inline

Using the wrong reference format causes Studio Pro to fail loading the model. Always check the metamodel reflection data to determine which format each property uses.

Storage Names vs Qualified Names

The $Type field in BSON must use the storageName, not the qualifiedName. These are often identical but not always:

qualifiedName (SDK)	storageName (BSON $Type)
`DomainModels$Entity`	`DomainModels$EntityImpl`
`DomainModels$Index`	`DomainModels$EntityIndex`

Using the wrong name causes TypeCacheUnknownTypeException when opening in Studio Pro. See Storage Names for more details.

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