curl --request POST \
  --url https://app.nex.ai/api/developers/v1/relationships \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "type": "one_to_many",
  "entity_definition_1_id": "123",
  "entity_definition_2_id": "456",
  "entity_1_to_2_predicate": "has",
  "entity_2_to_1_predicate": "belongs to"
}
'

{
  "id": "789",
  "type": "one_to_many",
  "entity_definition_1_id": "123",
  "entity_definition_2_id": "456",
  "entity_1_to_2_predicate": "has",
  "entity_2_to_1_predicate": "belongs to",
  "created_at": "2024-01-15T10:00:00Z"
}