Enforcing XOR (Either/Or) Fields in SurrealDB

Today's guest post is by Abdo (Abdelrahman Omar), a Rust software engineer from the great city of Alexandria in Egypt who has found SurrealDB ideal in simplifying a tech stack that would otherwise have needed Redis for a cache and a separate relational database on the side. Axum and Leptos are the two other tools Abdo enjoys using when putting together a fullstack Rust platform.

How to enforce XOR (Either/Or) Fields in SurrealDB

When designing a database schema, you may encounter situations where a record should contain one of two possible fields, but not both or neither. For example, a repository might be owned by either a user or an organisation, but never both simultaneously or neither. This is known as an XOR or “either/or” constraint.

Arc Relationship

This scenario is also known as an Arc Relationship in ER (Entity Relationship) modelling. In an ER Diagram, it is shown by drawing an arc across the relationship lines, hence the name arc relationship.

Modelling an arc relationship in SurrealDB

Here is the complete schema definition for our repository table to showcase how to enforce the XOR constraint.

Defining the schema

The schema is first set with a few DEFINE statements.

DEFINE TABLE repository SCHEMAFULL;

DEFINE FIELD name ON repository TYPE string;
-- Use `option<table_name>` to allow fields
-- to be omitted before the XOR check
DEFINE FIELD user         ON repository TYPE option<record<user>>;
DEFINE FIELD organization ON repository TYPE option<record<organization>>;

Defining the XOR check

After this comes the XOR check which uses a VALUE clause to set the value, and THROW to return an error if the XOR constraint is violated.

-- Fields in SurrealDB are represented as a key-value pair
-- `VALUE` is used to set the raw "value" of the field and omitting the "key"
DEFINE FIELD owner_type ON repository VALUE {
    -- 1. Check if NEITHER is set
    IF user = NONE AND organization = NONE {
        THROW "a repository requires either a user or an organization owner";
    }
    -- 2. Check if BOTH are set
    ELSE IF user != NONE AND organization != NONE {
        THROW "a repository cannot have both a user and an organization owner";
    };
    -- 3. (Optional) If validation passes, return the owner type
    RETURN IF user != NONE {
        'user';
    } ELSE {
        'organization';
    };
};

Testing

The four possible cases can be tested as follows.

-- Creating a repository with just a user owner
CREATE repository SET
    name = 'my_repo',
    user = user:abdo;

-- Creating a repository with just an organization owner
CREATE repository SET
    name = 'my_repo',
    organization = organization:abdo;

-- ERROR: Fails because BOTH user and organization are set.
CREATE repository SET
    name = 'hamadas_repo',
    user = user:hamada,
    organization = organization:hamada_org;
-- Throws: 'An error occurred: a repository cannot have both a user and an organization owner'

-- ERROR: Fails because NEITHER user nor organization is set.
CREATE repository SET
    name = 'hamada_repo';
-- Throws: 'An error occurred: a repository requires either a user or an organization owner'

You can play around with it more by copying and pasting the code into the Surrealist sandbox.

SurrealDB Cleaner Solution

A cleaner way approach was suggested by jimpex on surrealdb discord after the original blog was posted.

DEFINE FIELD owner ON repository TYPE record<user | organization>;
DEFINE FIELD owner_type ON repository VALUE record::tb(owner);

CREATE ONLY repository SET owner = user:andy;
CREATE ONLY repository SET owner = organization:dunder_mifflin;
CREATE ONLY repository SET owner = fail:test;

The SQL equivalent

For those coming from a relational background, this same logic is typically handled using a CHECK constraint in the table definition.

CREATE TABLE users (
    id INT PRIMARY KEY,
    name VARCHAR(255) NOT NULL
);

CREATE TABLE organizations (
    id INT PRIMARY KEY,
    name VARCHAR(255) NOT NULL
);

CREATE TABLE repository (
    id INT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    
    user_id INT,
    organization_id INT,
    
    FOREIGN KEY (user_id) REFERENCES users(id),
    FOREIGN KEY (organization_id) REFERENCES organizations(id),
    
    CONSTRAINT owner_xor_check CHECK (
        (user_id IS NOT NULL AND organization_id IS NULL)
        OR
        (user_id IS NULL AND organization_id IS NOT NULL)
    )
);

This approach moves validation logic out of your application and into the schema itself, so the database becomes the single source of truth.

I hope this helps you understand arc relationships and how to enforce XOR fields in SurrealDB. If you want to learn more about SurrealDB, you can check out the official documentation here.

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