# One-to-one relationship

> Create one-to-one relationships in IS Tools using a Unique Relation field plus IQL filters or rules to ensure each record links to exactly one parent.

Even thought tables in a data model are related in a way where one of them is the parent and the other the child, i.e., [one-to-many relationship](one_to_many_relationship.md), it is possible to enforce that each record will only have a single related record; for instance, let’s say we have the table "Project" related to the table "Purchase order", and it is wished that the purchase order assigned to a project cannot be used for any other project other than that one, like:

| Project | Purchase order |
| --- | --- |
| Proj-A | PO-10001 |
| Proj-B | PO-10002 |

If there were a Proj-C, the purchase orders PO-10001 and PO-10002 would not be available for it, because they are already taken, i.e., a unique purchase order for a unique project.

For that, create a field from the table "Project" (you may call it "PO"). As `Type`, choose `Relation`; as `Parent table`, choose `Purchase Order`. Then tick the `Unique` checkbox and save. Optionally, you may need to tick `Mandatory` as well, in case you need that no project is ever missing a purchase order. Then, for all the forms where you display a project with the "PO" Relation, apply the following `Filter` on the Relation field:

```
RecordName not in(PO from Project)
```

Make sure there are records available in the "Purchase order" table.

**Alternatively, in case you don’t have purchase orders available, but instead you wish to automatically create and relate** a purchase order once a project is created, go to [Rules manager](rules_manager.md), and create the following rule:

Trigger and Target table: Project

Trigger condition: Record keyword CREATED

Actions: Run IQL

```
let @relatedPO = insert "Purchase order"
  values RecordGroup = STANDARD

set PO = @relatedPO
```

Note that "RecordGroup = STANDARD" is just an example, but there you can add other fields of "Purchase order", read more [here](iql_in_rules.md#_create_new_records).

Then, go to Data model editor, select the "Purchase order" table, and set `Record name policy` to `Generated series`. Otherwise, you must provide a custom name for the `RecordName` in the rule above. For this alternative, you won’t need the `Unique` and `Mandatory` properties for the Relation field, nor the `Filter` in the Relation field in forms explained at the beginning of this topic. However, we recommend to have the Relation field as read-only in the forms, so no one can unassign what it was made in an automatic way.