How do I specify multiple "Prefer" headers?

Question

I'm currently building a Ruby SDK for the Graph API.

I'm working with delta queries on the message resource endpoints, specifically list-messages.

I need to specify two preferences utilizing the Prefer header(s):

1. allow unsafe HTML - "outlook.allow-unsafe-html"
2. maximum items per page/request - "odata.maxpagesize={num}"

There aren't any examples in the docs showing how this can be accomplished. I'm not sure whether they need to be concatenated into a single value or whether to specify multiple HTTP headers (or if this is even supported). Clarification here would be super helpful

Answer 1

According to RFC7240:

A client MAY use multiple instances of the Prefer header field in a single message, or it MAY use a single Prefer header field with multiple comma-separated preference tokens. If multiple Prefer header fields are used, it is equivalent to a single Prefer header field with the comma-separated concatenation of all of the tokens.

So you can use multiple Prefer header fields defining distinct preferences:

 POST /foo HTTP/1.1
 Host: example.org
 Prefer: respond-async, wait=100
 Prefer: handling=lenient
 Date: Tue, 20 Dec 2011 12:34:56 GMT

Or you may use a single Prefer header field with a comma-separated list of values:

 POST /foo HTTP/1.1
 Host: example.org
 Prefer: handling=lenient, wait=100, respond-async
 Date: Tue, 20 Dec 2011 12:34:56 GMT