How to create multiple database entries and have id of first inserted entry as parentId field for the rest entries?

I need to create multiple entries in db (I have array of data objects). The first entry should be the main, other entries should be the children of the first.

[entry1: {parentId: null}, entry2: {parentId: 'idOfInsertedEntry1'}, entry3: {parentId: 'idOfInsertedEntry1'}]

I can't use bulk creation as I can't get the id of first inserted entry during the process.
How to achieve the goal?

There is no easy way, you are going to need a custom route and a custom controller, and then splice..

const [root, ...childs] = entries;

const rootEntry = await strapi.entityService.create('api::...', { data: root });

const childsEntries = await Promise.all(
  children.map(async (child) => 
    await strapi.entityService.create('api::...', { data: {...child, parentId:rootEntry.id }})
));

return [ rootEntry, ...childsEntries ];

I've would also not recommend to use parentId, use parent as relation to own content type. ParentId give headaches when used with data transfer.

upd

so if you want to store something locally (entry specific) in strapi content type you could define a Service component, add it to your main entry as repeatable component (if you want search out of the box) or dynamicZone (if you want to have polymorphic but no search out of the box).

then example payload of mutation would be like this:

{
  data: {
    ...someStuff,
    services: [
    { ...someServiceStuff1 },
    { ...someServiceStuff2 } 
    ]
  }

and you would have this services bound specifically to that order. if you query you would do something like

{ where : { services: { param: someServiceParam } } }

parent and child

you do in your schema:

{
 ...,
"parent": {
      "type": "relation",
      "relation": "oneToOne",
      "target": "api::order.order"
    }
  ...
},

so then you can chain them like tree branches