feat(schema): support for union types

[vkarpov15]

Fix #10894. There's still work to do on this PR (TypeScript types, options testing, thinking about how to handle getters/setters on unions) but I wanted to get this out there for review.

Summary

type: 'Union' means a given path can be one of multiple types.

For example, in the following schema, purchased can be either a Boolean or a Date.

const schema = new Schema({
  purchased: {
    type: 'Union',
    of: [Boolean, Date]
  }
});

Order matters in of: Mongoose will loop through the Of schema types and take the first one that casts successfully, unless one of the schema types returns the original value, in which case Mongoose just takes the original value.

For example, in the above case, purchased: 1 would get converted to a boolean (true) because Mongoose can cast 1 -> true. But purchased: 2 would get converted to a Date, because 2 is not a value Mongoose can cast to a Boolean by default.

Examples

[hasezoey]

LGTM, though lint would need to be fixed and as Copilot already pointed out, the documentation for of still refers to maps.

[Copilot]

Pull Request Overview

This PR introduces support for union types in Mongoose schemas, allowing a schema path to be one of multiple types. It implements a new Union schema type with casting logic that tries each type in order and uses the first successful cast.

• Adds a new Union schema type that accepts an array of possible types in an of property
• Implements casting logic that preserves original values when possible and falls back to first successful cast
• Updates error messages to include string length information for better debugging

Reviewed Changes

Copilot reviewed 7 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/schema/union.js	Implements the core Union schema type with casting and setter logic
lib/schema/index.js	Exports the new Union schema type
lib/schema.js	Updates schema interpretation to handle Union types
lib/options/schemaUnionOptions.js	Defines options class for Union schema type
lib/schemaType.js	Adds string length information to validation error context
lib/error/messages.js	Updates error messages to include string length
test/schema.union.test.js	Comprehensive test suite for Union functionality
test/types/schema.test.ts	TypeScript type tests for Union schema inference

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[hasezoey]

This comment should likely mention that the property is also valid for Union now.