GraphQL

Serve the cats as a GraphQL schema: the Apollo driver generating the schema from decorated classes, an object type whose fields say exactly what they are, and a resolver with two queries.

Let the graph connect: a cat's owner and an owner's cats resolved only when a query asks for them, and the cats query taking a filter and paging through a class of arguments with defaults.

Change the cats through the schema: input types that describe what a client may send, validated by class-validator, a partial update derived from the create input, and Nest's exceptions as GraphQL errors.

Give values their real types: a registered enum the schema lists and checks, dates that travel as ISO strings and arrive as Date objects, and a custom Microchip scalar that validates what clients send and formats what they get.

Fields that hold more than one kind of object: an Animal interface that cats and dogs implement, with a field resolved once for both, a search that answers a union of cats, dogs and owners, and the resolveType functions that tell plain objects apart.

Protect the schema like any Nest API: the HTTP request that Nest's Apollo driver puts in every resolver's context, a guard and a parameter decorator that read it through GqlExecutionContext, Nest's exceptions as GraphQL errors, and formatError keeping an internal failure's details on the server.

Code that runs around a single field and around a whole request: a field middleware that masks a microchip for anyone who is not staff, and an Apollo plugin, a Nest provider with its dependencies, that names every operation in a response header and a log.

Push changes to clients as they happen: a PubSub provider, mutations that publish, a subscription that filters what each subscriber receives and one that turns a payload into a sentence, graded by specs that subscribe through the schema.

Write the end-to-end tests of the cats schema: a fresh application per test, queries and mutations posted to /graphql with supertest, answers compared as data and errors as codes, until four bugs planted in the code are all caught.