Expression

ZModel provides an expression system that allows you to construct complex computations from primitive constructs like literals and fields. Expressions are used extensively in defining access control policies and input validation rules.

Expression Types

Literal

String (e.g., "hello"), number (e.g., 42), and boolean (e.g., true, false) literals are supported. Strings can be enclosed in either single quotes (') or double quotes (").

List

Lists can be constructed using square brackets, e.g., [1, 2, 3] or ['a', 'b', 'c'].

Field Reference

Model's fields can be directly referenced by their names within the current model context.

Member Access

You can use the dot notation to access members of an expression that contains fields. E.g., author.name if author is a relation field. Member access expressions can be chained.

Unary Operator

The following operators are supported:

• ! (logical NOT): E.g., !true

Binary Operator

The following operators are supported:

• == (equal to): E.g., a == b
• != (not equal to): E.g., a != b
• > (greater than): E.g., a > b
• >= (greater than or equal to): E.g., a >= b
• < (less than): E.g., a < b
• <= (less than or equal to): E.g., a <= b
• && (logical AND): E.g., a && b
• || (logical OR): E.g., a || b

ZModel follows JavaScript's operator precedence and associativity rules.

Collection Predicate

Collection predicate expressions are used to express a boolean condition over a to-many relation field. The expression has the following three variants:

• relation?[CONDITION]

  Evaluates to true if at least one related record satisfies the condition. E.g., posts?[published == true].

• relation![CONDITION]

  Evaluates to true if all related records satisfy the condition. E.g., posts![published == true].

• relation^[CONDITION]

  Evaluates to true if no related records satisfy the condition. E.g., posts^[published == true].

The CONDITION is an expression under the context of the relation, meaning that fields referenced in the condition are resolved against the related model. You can use this keyword to "escape" and refer to the fields belonging to the model where the rule is defined.

You can nest collection predicate expressions to traverse multiple levels of to-many relations. It's possible that field names become ambiguous due to nesting (i.e., outer model and inner model both have fields with the same name). To help disambiguate, collection predicate expression allows you to define a "binding variable" to explicitly denote the element of the current collection. For example:

// are there published posts that have published comments?
posts?[p, p.published && comments?[c, c.published]]

in Operator

The in keyword can be used to check if a value exists in a list. E.g., value in [1, 2, 3].

Function Calls

Functions can be called using the standard syntax: functionName(arg1, arg2, ...). See Functions for the list of available functions.

Null

The null keyword represents a null value expression.

This

The this keyword refers to the current entity of the model.