I want to save data in CouchDB documents and as I am used to doing it in RDBMS. I want to create a field which can only contain a unique value in the database. If I now save a document and there is already a document with unique key I expect an error from CouchDB.
I guess I can use the document ID and replace the auto generated doc-id by my value, but is there a way to set other field as unique key holder. Any best practice regarding unique keys?
As you said, the generated _id is enforced as unique. That is the only real unique constraint in CouchDB, and some people use it as such for their own applications.
However, this only applies to a single CouchDB instance. Once you start introducing replication and other instances, you can run into conflicts if the same _id is generated on more than 1 node. (depending on how you generate your _ids, this may or may not be a problem)
As Dominic said, the _id is the only parameter that is almost assured to be unique. One thing that is sure is that you have to design your "database" in a different way. Keep in mind that the _id will be database wide. You will be able to have only one document with this _id.
The _id must be a string, which means you can't have an array or a number or anything else.
If you want to make the access public, you'll have to think about how to generate your id in a way that it won't mess with your system.
I came up with ids that looked like that:
"email:email@example.com"
It worked well in my particular case to prevent people from creating multiple auth on the same email. But as Documinic said, if you have multiple masters, you'll have to think about possible conflicts.