# Incorrect relation matching when using custom ID field
**URL:** https://community.fibery.io/t/incorrect-relation-matching-when-using-custom-id-field/10775
**Category:** API & Programming
**Created:** [April 27, 2026, 11:56am UTC](https://community.fibery.io/t/incorrect-relation-matching-when-using-custom-id-field/10775 "2026-04-27T11:56:08Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@teibrich](https://community.fibery.io/u/teibrich)
#### Post date: [April 27, 2026, 11:56am UTC](https://community.fibery.io/t/incorrect-relation-matching-when-using-custom-id-field/10775/1 "2026-04-27T11:56:08Z")
Hi there,
I’m experiencing an issue with the Fibery Integration API where entities are being linked incorrectly despite sending correct data. Not sure if this is an API issue or if I just didn’t quite understand how the intended usage is.
**Setup:**
- We’re syncing data from an external system (Jira Assets) to Fibery
- Using the `/api/v1/synchronizer/data` endpoint with `synchronizationType: 'full'`
- Entities have a custom `id` field (using keys like “ES-123”, “ES-456”) instead of UUIDs
- Relations are configured with `targetFieldId: 'id'` to match against this custom field
**The Problem:**
When syncing a “Measure” entity (let’s call it “M-1312”) that should only be linked to one “School” entity (“S-49”), Fibery is incorrectly linking it to multiple schools (both “S-4” and “S-49”).
**What we’re sending:**
```json
{
"id": "M-1312",
"name": "Measure 1312",
"schulen": ["S-49"]
}
```
**Target entities in Fibery:**
```json
// School S-4
{
"id": "S-4",
"name": "School 4"
}
// School S-49
{
"id": "S-49",
"name": "School 49"
}
```
**Expected behavior:**
M-1312 should only be linked to S-49 (since `schulen` only contains `["S-49"]` and the relation uses `targetFieldId: 'id'` for exact matching).
**Actual behavior:**
M-1312 is linked to BOTH S-4 and S-49.
**Schema configuration:**
```json
"schulen": {
"type": "array[text]",
"relation": {
"cardinality": "many-to-many",
"targetType": "school",
"targetFieldId": "id"
}
}
```
**Questions:**
1. Does Fibery perform any fuzzy matching on the `id` field (e.g., prefix matching)?
2. Should the `id` field be of a specific format?
3. Is there a known issue with using non-UUID values in the `id` field for relation matching?
Any insights would be appreciated!
Alex
---
### Author:  [@Chr1sG](https://community.fibery.io/u/Chr1sG)
#### Post date: [April 27, 2026, 1:17pm UTC](https://community.fibery.io/t/incorrect-relation-matching-when-using-custom-id-field/10775/2 "2026-04-27T13:17:52Z")
Fibery uses `contains` for matching, so indeed, if you are matching on ‘S-49’ then both ‘S-49’ and ‘S-4’ will be linked.
I’d suggest doing some kind of hashing or string wrapping of the values prior to matching, so that you can avoid this.
---
### Author:  [@teibrich](https://community.fibery.io/u/teibrich)
#### Post date: [April 27, 2026, 1:40pm UTC](https://community.fibery.io/t/incorrect-relation-matching-when-using-custom-id-field/10775/3 "2026-04-27T13:40:42Z")
Thanks a lot for your quick reply! I will create a UUID and use that.
From an implementation perspective it really does not matter, but I don’t find it obvious that you use `contains` to match two IDs. If you have the option, perhaps add a note to your documentation 😉
Alex