Xpath Constraints

---
name: xpath-constraints
description: "XPath constraint syntax for MDL — retrieve WHERE clauses, page data sources, and row-level entity access, including association paths and functions. Use when writing or debugging any XPath in a project."
---

# XPath Constraints in MDL

This skill provides reference for writing XPath constraint expressions in MDL RETRIEVE statements, page data sources, and security rules.

## When to Use This Skill

- Writing `retrieve ... where [xpath]` statements in microflows
- Writing `database from entity where [xpath]` in page data sources
- Writing `grant ... where 'xpath'` for row-level entity access
- Debugging XPath parsing or serialization issues

## XPath vs Mendix Expressions

**Critical distinction**: XPath constraints (inside `[...]`) use different syntax from Mendix expressions (in SET, IF, DECLARE, etc.):

| Feature | XPath `[...]` | Mendix Expression |
|---------|---------------|-------------------|
| Path separator | `/` (always path traversal) | `/` (also division) |
| Boolean ops | lowercase: `and`, `or`, `not()` | `and`, `or`, `not` |
| Negation | `not(expr)` function | `not expr` |
| Empty check | `= empty`, `!= empty` | `= empty` |
| Token quoting | `'[%CurrentUser%]'` (quoted) | `[%CurrentUser%]` (unquoted) |
| Nested filter | `Assoc/entity[pred]` | Not applicable |
| Arithmetic on value | **not supported** — pre-compute into a variable | `+`, `-`, `*`, `div`, `mod` |

> **XPath constraints cannot compute values.** `where [Seq = $Game/MoveSeq + 1]`
> is a parse error (Mendix XPath has no arithmetic on the value side). Compute the
> value first, then compare against the variable:
> ```mdl
> declare $Next integer = $Game/MoveSeq + 1;
> retrieve $M from Mod.Move where [Seq = $Next] first;
> ```
> `mxcli check` explains this and shows the workaround when it sees `+`/`*`/`div`/
> `mod` inside a constraint.

> **A negative literal is fine, though.** A leading `-` on a number is a value,
> not arithmetic, and needs no quoting:
> ```mdl
> retrieve $L from Mod.T where [Amount > -7];
> retrieve $L from Mod.T where [Amount <= -12.5 and Code != 'X'];
> ```

> **Date arithmetic is not available in XPath.** `addDays()`, `addMonths()` and
> friends are *Mendix expression* functions — using one in a constraint fails the
> build with `CE0161` regardless of its arguments. For relative dates use the
> date tokens (`[%CurrentDateTime%]`, `[%BeginOfCurrentDay%]`, …), or compute the
> cut-off in a variable first and compare against that:
> ```mdl
> declare $Cutoff datetime = addDays([%CurrentDateTime%], -7);
> retrieve $L from Mod.T where [DueDate > $Cutoff];
> ```

## Syntax Reference

### Simple Comparisons

```mdl
retrieve $Orders from Module.Order
  where [State = 'Completed'];

retrieve $Active from Module.Customer
  where [IsActive = true];

retrieve $Recent from Module.Order
  where [OrderDate != empty];

retrieve $HighValue from Module.Order
  where [TotalAmount >= $MinAmount];
```

Operators: `=`, `!=`, `<`, `>`, `<=`, `>=`

> **Inline vs quoted form.** The inline bracket form `where [State = 'Completed']`
> is preferred. The quoted form — `where '[State = ''Completed'']'`, with internal
> single quotes doubled (`''`) — is also accepted for `retrieve` and datasource
> `where` clauses, and now stores the identical constraint (it un-escapes the `''`
> and strips the outer quotes). Don't double-bracket: write either `[...]` or
> `'[...]'`, not both.

### Boolean Logic

```mdl
-- AND
where [State = 'Completed' and IsPaid = true]

-- OR
where [State = 'Pending' or State = 'Processing']

-- Grouped
where [State = 'Completed' and ($IgnorePaid or IsPaid = true)]

-- NOT
where [not(IsPaid)]
where [not(contains(Name, 'demo'))]
```

## How a Constraint Is Laid Out on Disk

MDL keeps a constraint on one line; how it is **stored** is decided by mxcli, not
by the whitespace you type. A constraint is rebuilt from its parse tree on every
write, so there is no original formatting to keep — instead the layout is derived
from the expression:

- **80 columns or fewer** → stored exactly as written, on one line. This is the
  common case, and it means adding this changed nothing about existing projects.
- **Longer** → broken at its top-level `and`/`or` joints, one clause per line,
  the operator leading each continuation line. Where `and` and `or` meet, the
  `and` runs get explicit parentheses — Mendix binds `and` tighter, and a filter
  is being broken up precisely because it had stopped being obvious at a glance.
- **Nothing to break on** (one long comparison, one long association path) →
  left whole and over width. Cutting it anywhere else would not be valid XPath.

```
-- authored (one line, 141 characters)
where [Archived = false and Status = 'Open' and Priority = 'High' and Category = 'Electrical' and Severity > 3 and ReportedOn > '[%BeginOfCurrentDay%]']

-- stored, and what Studio Pro's XPath editor shows
[
  Archived = false
  and Status = 'Open'
  and Priority = 'High'
  and Category = 'Electrical'
  and Severity > 3
  and ReportedOn > '[%BeginOfCurrentDay%]'
]
```

`DESCRIBE` puts it back on one line, so a description reads the way it always
has and re-executing it re-derives the same stored text — the unit is reported
`Unchanged`. A constraint mxcli cannot parse is stored exactly as given rather
than reformatted.

This applies to page datasources, `retrieve … where` in microflows, and entity
access rules alike.

### Association Path Traversal

Bare association paths (without `$variable` prefix) navigate through the domain model:

```mdl
-- Single-hop: filter by associated object
where [Module.Order_Customer = $Customer]

-- Multi-hop: traverse through associations
where [Module.Order_Customer/Module.Customer/Name = $CustomerName]

-- Existence check: has an associated object
where [Module.Order_Customer/Module.Customer]

-- Negated existence: has NO associated object
where [not(Module.Order_Customer/Module.Customer)]
```

**Rule**: Always use the fully qualified association name (`Module.AssociationName`).

> **A bare association name is now caught before the build (MDL-XPATH01).**
> `[Order_Customer = $currentUser]` used to pass `mxcli check --references`, get
> written by `exec`, and only fail at the build with *"Error(s) in XPath
> constraint"* (**CE0161**) — which is the expensive shape, because `exec` cannot
> roll back and stops with the model half-updated. `check` now names the
> association and the qualified spelling to use instead. It fires only when the
> bare name is not an attribute of the constrained entity **and** is a known
> association, so attributes stay bare and XPath functions are never touched.

> **`= empty` / `!= empty` do not work on associations (CE0161 / MDL047).**
> `empty` tests *attribute* nullability only. To test whether an object *has no*
> associated object, use negated existence: `[not(Module.Order_Customer/Module.Customer)]`
> — **not** `[Module.Order_Customer = empty]`; to test that it *has* one, the path
> itself: `[Module.Order_Customer/Module.Customer]` — **not** `!= empty`.
> `mxcli check` flags both forms as **MDL047** before the build does.

### Variable Paths

```mdl
-- Compare attribute via variable path
where [Module.Assoc/Module.Entity/Name = $Variable/Name]

-- Variable on right side
where [Name = $currentObject/SearchString]
```

### Nested Predicates

Filter intermediate path steps with inline `[predicate]`:

```mdl
-- Only lines of completed orders
where [Module.OrderLine_Order/Module.Order[State = 'Completed']]

-- Nested predicate with further traversal
where [Module.OrderLine_Order/Module.Order[State = 'Active']/Module.Order_Category/Module.Category/Name = $CategoryName]

-- reversed() path modifier (traverse association in reverse direction)
where [System.grantableRoles[reversed()]/System.UserRole/System.UserRoles = '[%CurrentUser%]']
```

### Functions

```mdl
-- String search
where [contains(Name, $SearchStr)]
where [starts-with(Name, $Prefix)]
where [not(contains(Name, 'demo'))]

-- Boolean functions
where [IsActive = true()]
where [Displayed = false()]
```

Supported functions: `contains()`, `starts-with()`, `not()`, `true()`, `false()`

The expression functions `startsWith()` / `endsWith()` are not XPath: in a
constraint they are CE0161, and `check` reports them as **MDL091**. A member the
entity does not have is a reference error in `check -p`, and so is a system
member written the way `describe` prints the attribute: XPath spells it
`createdDate`, `changedDate`, `owner`, `changedBy` — `[CreatedDate > …]` is CE0161.

### Tokens

Mendix tokens provide runtime values. In an XPath constraint a token used as a
value is stored quoted as `'[%Token%]'` (Studio Pro requires this, or it reports
CE0161). mxcli quotes it for you whether you write the bare or quoted form:

```mdl
-- Both store identically as '[%CurrentDateTime%]' and pass mx check
where [OrderDate < [%CurrentDateTime%]]
where [OrderDate < '[%CurrentDateTime%]']
where [System.owner = '[%CurrentUser%]']
```

> **Tokens are typed.** `[%CurrentUser%]` is a **User** reference — compare it only
> to an association to System.User (e.g. `System.owner`), never to a String/other
> attribute (`[Title = '[%CurrentUser%]']` is a type error → CE0161).
> `[%CurrentDateTime%]` compares to DateTime attributes, etc.

> **`System.owner` / `System.changedBy` must be enabled** on the entity before you
> can reference them in XPath, or Studio Pro reports CE0161. Enable with
> `alter entity Module.Entity add attribute owner: autoowner;` (mxcli's
> `check --references` flags this). Same for `changedBy`/`changedDate`/`createdDate`.

Common tokens: `[%CurrentUser%]`, `[%CurrentDateTime%]`, `[%CurrentObject%]`, `[%UserRole_RoleName%]`, `[%DayLength%]`

### ID Pseudo-Attribute

The `id` pseudo-attribute compares object identity (GUID):

```mdl
where [id = $currentUser]
where [id != $existingObject]
where [id = '[%CurrentUser%]']
```

## Usage Contexts

### RETRIEVE in Microflows

```mdl
retrieve $Results from Module.Entity
  where [IsActive = true and State = 'Ready']
  sort by Name asc
  limit 100;
```

The expression inside `[...]` is parsed as XPath and stored in BSON as the `XpathConstraint` field.

### Page Data Sources

```mdl
datagrid dg (
  datasource: database from Module.Entity where [State != 'Cancelled'] sort by Name asc
) {
  column (attribute: Name, caption: 'Name')
}
```

Multiple bracket constraints can be chained. Consecutive brackets without an operator are treated as AND (standard Mendix XPath):

```mdl
-- Consecutive brackets (implicit AND) — standard Mendix XPath syntax
datasource: database from Module.Entity where [IsActive = true][Stock > 0]

-- Explicit AND: same result
datasource: database from Module.Entity where [IsActive = true] and [Stock > 0]

-- Mix with OR: combines into single bracket
datasource: database from Module.Entity where [IsActive = true] or [Stock > 10]
```

### GRANT Entity Access (Security)

Security rules take the XPath in brackets, like every other XPath, so quotes
inside it are written once:

```mdl
mdl 1;
grant read *, write * on entity Module.Entity to Module.Role
  where [System.owner = '[%CurrentUser%]'];
```

Sibling groups (`where [a][b]`) are kept as one constraint. The old quoted form
`grant Module.Role on Module.Entity (...) where '[...]'` still parses, warns
MDL-DEPR030, and `mxcli fmt --upgrade` rewrites it.

## Enumeration Attributes

**Critical**: XPath constraints are translated to database SQL WHERE clauses at runtime. The database stores enum values as plain strings (the value key), not qualified names. This means:

- `[Status = 'Open']` — always valid: direct string literal match
- `[Status = Module.OrderStatus.Open]` — also valid: mxcli converts to `'Open'` in BSON automatically

Both forms are accepted by mxcli in the write direction. `DESCRIBE MICROFLOW` always shows the qualified name form for readability, even though BSON stores `'Open'`.

**Do NOT use qualified names in expression context (IF, SET, DECLARE) for comparisons** — those contexts use a different form. See `write-microflows` "Enumeration Comparisons" section.

```mdl
-- Preferred (mxcli converts to 'Open' in BSON):
retrieve $OpenOrders from Module.Order
  where [Status = Module.OrderStatus.Open];

-- Also accepted (stored as-is):
retrieve $OpenOrders from Module.Order
  where [Status = 'Open'];

-- NOT equal
retrieve $Active from Module.Order
  where [Status != Module.OrderStatus.Cancelled];

-- OR across multiple enum values
retrieve $InProgress from Module.Order
  where [Status = Module.OrderStatus.Open or Status = Module.OrderStatus.Processing];

-- Enum combined with other predicates
retrieve $Results from Module.Order
  where [Status = Module.OrderStatus.Completed and TotalAmount >= $MinAmount];
```

### Troubleshooting silent empty results with enums

If a RETRIEVE returns empty unexpectedly when filtering by an enum attribute:
1. Check the **value key** (not caption) — the key is what's stored in the DB column. Check with `DESCRIBE ENUMERATION Module.EnumName`.
2. Keys are **case-sensitive**: `'open'` ≠ `'Open'`.
3. Confirm the attribute type is actually an enumeration and not a string — `DESCRIBE ENTITY Module.EntityName`.

## Common Patterns

### Parameterized Search

```mdl
mdl 1;
create microflow Module.Search ($query: string, $ActiveOnly: boolean)
returns boolean
begin
  retrieve $Results from Module.Customer
    where [($ActiveOnly = false or IsActive = true)
      and (contains(Name, $query) or contains(Email, $query))];
  return true;
end;
```

### Date Range Filter

```mdl
retrieve $Orders from Module.Order
  where [OrderDate >= $StartDate and OrderDate <= $EndDate];
```

### Optional Filters (empty = skip)

```mdl
retrieve $Orders from Module.Order
  where [($Category = empty or Module.Order_Category = $Category)
    and ($State = empty or State = $State)];
```

### Owner-Based Security

```mdl
-- In microflow
retrieve $MyItems from Module.Item
  where [System.owner = '[%CurrentUser%]'];

-- In security rule
grant read * on entity Module.Item to Module.User where [System.owner = '[%CurrentUser%]'];
```

## Validation

Always validate XPath syntax before execution:

```bash
# Syntax check (no project needed)
./bin/mxcli check script.mdl

# with reference validation (needs project)
./bin/mxcli check script.mdl -p app.mpr --references
```

## Troubleshooting

| Issue | Cause | Fix |
|-------|-------|-----|
| `mismatched input` on keyword | Attribute name is a reserved word | This is handled — `xpathWord` accepts any keyword as identifier |
| Token not quoted in BSON | Token in Mendix expression context | Use `[...]` bracket syntax for XPath, not bare expression |
| `CE0111` path error | Missing module prefix on association | Use `Module.AssociationName`, not just `AssociationName` |
| `CE0161` XPath constraint error | Qualified name used for non-enum or wrong format | Use string literal `'Value'` or qualified name `Module.Enum.Value`; mxcli converts automatically |
| `not` parsed as keyword | Using `not` (uppercase) in XPath | XPath uses lowercase `not()` as a function |
| Retrieve returns empty for enum filter | String literal value key mismatch | Key is case-sensitive; verify with `DESCRIBE ENUMERATION Module.Name` |