Apollo/GraphQL field type for object with dynamic keys

GraphQL relies on both the server and the client knowing ahead of time what fields are available available for each type. In some cases, the client can discover those fields (via introspection), but for the server, they always need to be known ahead of time. So to somehow dynamically generate those fields based on the returned data is not really possible.

You could utilize a custom JSON scalar (graphql-type-json module) and return that for your query:

type Query {
  persons(area: String): JSON
}

By utilizing JSON, you bypass the requirement for the returned data to fit any specific structure, so you can send back whatever you want as long it's properly formatted JSON.

Of course, there's significant disadvantages in doing this. For example, you lose the safety net provided by the type(s) you would have previously used (literally any structure could be returned, and if you're returning the wrong one, you won't find out about it until the client tries to use it and fails). You also lose the ability to use resolvers for any fields within the returned data.

But... your funeral :)

As an aside, I would consider flattening out the data into an array (like you suggested in your question) before sending it back to the client. If you're writing the client code, and working with a dynamically-sized list of customers, chances are an array will be much easier to work with rather than an object keyed by id. If you're using React, for example, and displaying a component for each customer, you'll end up converting that object to an array to map it anyway. In designing your API, I would make client usability a higher consideration than avoiding additional processing of your data.

You can write your own GraphQLScalarType and precisely describe your object and your dynamic keys, what you allow and what you do not allow or transform.

See https://graphql.org/graphql-js/type/#graphqlscalartype

You can have a look at taion/graphql-type-json where he creates a Scalar that allows and transforms any kind of content:

https://github.com/taion/graphql-type-json/blob/master/src/index.js