routes/index.js

index.js

...

const populateRelated = (model, fieldList) => new Promise((resolve) => {
  keystone.populateRelated(model, fieldList, () => {
    resolve(model)
  })
})

...

  keystoneApp.get('/api/core/entity', async (req, res, next) => {
    try {
      const CoreEntity = keystone.list('CoreEntity')

      const result = await populateRelated(
        await CoreEntity.model.find().exec(),
        ['fields']
      ).map((doc) => {
        const fields = doc.fields.map((fieldDoc) => fieldDoc.toJSON())
        return {
          ...doc.toJSON(),
          fields,
        }
      })

      res.json(result)
    } catch (err) {
      if (err) throw err
    }
  })

...