Constraints are an EdgeDB mechanism that provides fine-grained control over which data is considered valid. A constraint may be defined on a scalar type, an object type, a concrete link, or a concrete property. In case of a constraint on a scalar type, the subjects of the constraint are the instances of that scalar, thus the values that the scalar can take will be restricted. Whereas for link or property constraints the subjects are the targets of those links or properties, restricting what objects or values those links and properties may reference. The subject of a constraint can be referred to in the constraint expression as __subject__.

The standard library defines the following constraints:

constraint
std::one_of
std::one_of(VARIADIC members: anytype)

Specifies the list of allowed values directly.

Example:

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scalar type Status extending str {
    constraint one_of ('Open', 'Closed', 'Merged');
}
constraint
std::expression
std::expression on (expr)

Arbitrary constraint expression.

Example of using an expression constraint to create a custom scalar:

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scalar type starts_with_a extending str {
    constraint expression on (__subject__[0] = 'A');
}

Example of using an expression constraint based on a couple of object properties to restrict maximum magnitude for a vector:

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type Vector {
    required property x -> float64;
    required property y -> float64;
    constraint expression on (
        __subject__.x^2 + __subject__.y^2 < 25
    );
}
constraint
std::max_value
std::max_value(max: anytype)

Specifies the maximum value for the subject.

Example:

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scalar type max_100 extending int64 {
    constraint max_value(100);
}
constraint
std::max_ex_value
std::max_ex_value(max: anytype)

Specifies the maximum value (as an open interval) for the subject.

Example:

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scalar type maxex_100 extending int64 {
    constraint max_ex_value(100);
}
constraint
std::max_len_value
std::max_len_value(max: int64)

Specifies the maximum length of subject string representation.

Example:

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scalar type Username extending str {
    constraint max_len_value(30);
}
constraint
std::min_value
std::min_value(min: anytype)

Specifies the minimum value for the subject.

Example:

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scalar type non_negative extending int64 {
    constraint min_value(0);
}
constraint
std::min_ex_value
std::min_ex_value(min: anytype)

Specifies the minimum value (as an open interval) for the subject.

Example:

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scalar type positive_float extending float64 {
    constraint min_ex_value(0);
}
constraint
std::min_len_value
std::min_len_value(min: int64)

Specifies the minimum length of subject string representation.

Example:

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scalar type four_decimal_places extending int64 {
    constraint min_len_value(4);
}
constraint
std::regexp
std::regexp(pattern: str)

Specifies that the string representation of the subject must match a regexp.

Example:

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scalar type LettersOnly extending str {
    constraint regexp(r'[A-Za-z]*');
}

See here for more details on regexp patterns.

constraint
std::exclusive

Specifies that the link or property value must be exclusive (unique).

When applied to a multi link or property, the exclusivity constraint guarantees that for every object, the set of values held by a link or property does not intersect with any other such set in any other object of this type.

This constraint is only valid for concrete links and properties. Scalar type definitions cannot include this constraint.

Example:

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type User {
    # Make sure user names are unique.
    required property name -> str {
        constraint exclusive;
    }

    # Make sure none of the "owned" items belong
    # to any other user.
    multi link owns -> Item {
        constraint exclusive;
    }
}

Sometimes it’s necessary to create a type where each combination of properties is unique. This can be achieved by defining an exclusive constraint for the type, rather than on each property:

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type UniqueCoordinates {
    required property x -> int64;
    required property y -> int64;

    # Each combination of x and y must be unique.
    constraint exclusive on ( (.x, .y) );
}

In principle, many possible expressions can appear in the on (<expr>) clause of the exclusive constraint with a few caveats:

  • The expression can only contain references to the immediate properties or links of the type.

  • No backlinks or long paths are allowed.

  • Only Immutable functions are allowed in the constraint expression.

This constraint also has an additional effect of creating an implicit index on the link or property. This means that in the above example there’s no need to add explicit indexes for the name property.

Constraint SDL, DDL, and introspection.

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