GraphQL Resolvers - why it is populating only if I have method find in resolver?

Question

Today I witnessed something very strange for me. Lets assume I have file case.graphql:

 extend type Query {
    getSingleCase(caseId: String): Case
    }

    type Case {
      _id: String!
      history: [CaseHistory]
    }
    
    type CaseHistory {
      _id: String
      oldDescription: String
    }

So respectfully I have 2 files - CaseModel.ts and CaseHistoryModel.ts:

CaseModel:

import { Schema, model } from "mongoose";

const caseSchema = new Schema({
  history: [{ type: Schema.Types.ObjectId, ref: "CaseHistory" }],
});

export default model("Case", caseSchema);

CaseHistoryModel:

import { Schema, model } from "mongoose";

const caseHistory = new Schema({
  oldDescription: {
    type: String,
  }
});

export default model("CaseHistory", caseHistory);

In my caseResolver.ts:

 Query:{
    getSingleCase: async (
          parentValue: any,
          args: { caseId: String },
          context: any
        ) => {
        
          return await CaseModel.findById(args.caseId)
            .populate({
              path: "history",
            })
    }
}

I try to populate history for the case. But this is returning me:

"MissingSchemaError: Schema hasn't been registered for model "CaseHistory".",

But if I add another query in the resolver:

Query:{
    getHistory: async (parentValue: any, args: any, context: any) => {
      const casehistory = await CaseHistoryModel.find();
      return casehistory;
    },
    getSingleCase: async (
      parentValue: any,
      args: { caseId: String },
      context: any
    ) => {
          return await CaseModel.findById(args.caseId)
         .populate({
             path: "history",
          })
    }

Why is that behaviour? Why I have to add this query called getHistory and even thought I am not using it anywhere, it works with it. Is it because I am engaging usage of CaseHistoryModel and as if I am using it in resolver it is read and correctly applied to schema?