I write to the json import endpoint a list of contacts in "import_data" and one list id in "list_ids". Let's say one of those contacts is already in Constant Contact. That contact is a part of the "Second List" list. In my json import, I'm re-importing that contact with only one list in the "list_ids": "First List".
If I were to run the import, would the contact's membership to "Second List" be removed?
Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
Both of V3's Bulk Import endpoints (JSON and CSV) work similarly to uploading a file via the Website UI, in that it will only add list memberships (rather than remove any), and will only update contact fields that are provided via the file based on the email address of the contact.
If you were to add a contact via one of the Bulk Import endpoints to list C, but they were already on lists A and B, they would now be on lists A, B, and C.
Import Contacts using a JSON Payload
Import Contacts using a CSV File
Please have a look and let us know if you have any other questions!