Discriminator validation issue

[pnoebauer]

High-Level Description

This pull request illustrates a bug in the LoopBack framework causing discriminators to not work. The issue is due to LoopBack-Rest invoking the openapi-schema-to-json-schema library function incorrectly under loopback-next/packages/rest/src/validation/request-body.validator.ts, which results in the removal of OpenAPI properties such as discriminator.

Changes Made

Bug Illustration:
The bug is located in loopback-next/packages/rest/src/validation/request-body.validator.ts#L87.
Added comments and TODO notes in the pr to highlight the issue.
Demonstrated the bug in the /examples/validation-app by adding a controller, models, and tests.
Code Changes:
In loopback-next/packages/rest/src/validation/request-body.validator.ts:

function convertToJsonSchema(openapiSchema: SchemaObject) {
  /*
    loopback does not pass the options arg to function toJsonSchema(schema, options?),
    which is part of the openapi-schema-to-json-schema library;
    by default toJsonSchema() removes discriminators, which is why they do not work
    in loopback
  */
  // TODO: fix below bug; toJsonSchema requires the options object to enable discriminators
  const jsonSchema = toJsonSchema(openapiSchema);
  // NOTE: replacing above line with below fixes discriminators not working in loopback and all tests pass
  // const jsonSchema = toJsonSchema(openapiSchema, {
  //   keepNotSupported: ['discriminator'],
  // });
}

Example Application:
Created an example application under /examples/validation-app to reproduce the issue.
Added a controller and models to showcase the problem.
Included tests that demonstrate the expected error messages if discriminators worked correctly.

[aaqilniz]

Hi, @pnoebauer. Can you please uncomment the actual change we need to fix the issue?

[pnoebauer]

Hi, @pnoebauer. Can you please uncomment the actual change we need to fix the issue?

Hi, @pnoebauer. Can you please uncomment the actual change we need to fix the issue?

Hi, sure! I have just pushed another commit with the comments removed and a possible bug fix.

[pnoebauer]

@aaqilniz @rmg @klassicd @dhmlau could you please review this pr

please advise if any changes are needed