budibase/packages/core/src/templateApi/canDeleteIndex.js

import { 
  findRoot, 
  getFlattenedHierarchy,
  fieldReversesReferenceToIndex,
  isRecord
} from "./hierarchy"
import { $ } from "../common"
import { map, filter, reduce } from "lodash/fp"

export const canDeleteIndex = indexNode => {
  const flatHierarchy = $(indexNode, [
    findRoot,
    getFlattenedHierarchy
  ])

  const reverseIndexes = $(flatHierarchy,[
    filter(isRecord),
    reduce((obj, r) => {
      for (let field of r.fields) {
        if (fieldReversesReferenceToIndex(indexNode)(field)) {
          obj.push({ ...field, record:r })
        }
      }
      return obj
    },[]),
    map(f => `field "${f.name}" on record "${f.record.name}" uses this index as a reference`)
  ])

  const lookupIndexes = $(flatHierarchy,[
    filter(isRecord),
    reduce((obj, r) => {
      for (let field of r.fields) {
        if (field.type === "reference" 
            && field.typeOptions.indexNodeKey === indexNode.nodeKey()) {
          obj.push({ ...field, record:r })
        }
      }
      return obj
    },[]),
    map(f => `field "${f.name}" on record "${f.record.name}" uses this index as a lookup`)
  ])

  const errors = [
    ...reverseIndexes,
    ...lookupIndexes
  ]

  return {
    canDelete: errors.length === 0,
    errors
  }
}