How can I build a nested list using builder pattern?

Question

I am trying to make a query builder for GraphQL syntax, by using a builder pattern. I already did it for the filtering part:

I want to programatically make a way to say what data I want to get back from the query. (NOTE: before the title part of the query was hardcoded into the QueryBuilder.

So for that I made a class called Entity which has a list of itself as an attribute.

class Entity(
        private val entity: EntityType,
        private val entities: MutableList<Entity> = mutableListOf()
) {
    override fun toString(): String {
        return if (this.entities.isEmpty()) {
            entity.str
        } else {
            this.entity.str + "{" + this.entities.onEach {
                it.toString()
            }.joinToString(" ") + "}"
        }
    }
}

And I got it to work. But to build the structure. then I have to append to the list and "visit" the list of the entity to visit the entities in that list.

For example if I want to build this query:

{
  events(place:"Somewhere", area:"The place"){
    title
    location{
      coordinates{
        longitude
        latitude
      }
    }
  }
}

Then I need to go 2 levels down in the layers of the Entity, And the way I am doing it now, then my code is getting very wide.

fun main() {
    val e1 = listOf<Entity>(
            Entity(EntityType.TITLE),
            Entity(EntityType.LOCATION,
                    mutableListOf(
                            Entity(EntityType.COORDINATES,
                                    mutableListOf(
                                            Entity(EntityType.LONGITUDE),
                                            Entity(EntityType.LATITUDE)
                                    )
                            )
                    )
            )
    )

    val q1 = Query.Builder()
            .filter(FilterType.PLACE, "Somewhere")
            .filter(FilterType.AREA,"The place")
            .add(e1)
            .build()
    println(q1.query)
}

So I was thinking to use builder pattern. Or something else which will be smarter.

Thank you in advance for your help, and have a nice day.

BTW: I am using following enums:

enum class EntityType(val str: String) {
    EVENTS("events"),
    TITLE("title"),
    LOCATION("location"),
    COORDINATES("coordinates"),
    LONGITUDE("longitude"),
    LATITUDE("latitude"),
}

enum class Filter(val str: String) {
    PLACE("place"),
    AREA("area"),
}

And the rest of my code looks like this:

class Filter {
    var filters = mutableMapOf<FilterType, Any>()
    override fun toString(): String {
        return if (filters.isNotEmpty()) {
            "(" + filters.map {
                if (it.value is Int || it.value is Long) {
                    it.key.str + ":" + it.value + ","
                } else {
                    it.key.str + ":\"" + it.value + "\","
                }
            }.joinToString(" ").dropLast(1) + ")"
        } else {
            ""
        }
    }
}

class Query private constructor(val query: String) {
    class Builder {
        private lateinit var entities: List<Entity>
        private var filter = Filter()

        fun filter(key: FilterType, value: Any) = apply {
            this.filter.filters[key] = value
        }

        fun add(entities: List<Entity>) = apply {
            this.entities = entities
        }

        private fun collect(root:String):String {
            return "{$root$filter{${entities.joinToString(" ")}}}"
        }

        fun build(): Query {
            return Query(collect("events"))
        }
    }

Answer 1

Take a look to Kotlin Type-Safe Builders documentation. You can find an example on how to do it for HTML