Question: I want to get related videos only from the uploader's channel, but it looks like a search with `relatedToVideoId` will ignore `channelId` when specified.

E.g. `https://www.googleapis.com/youtube/v3/search?channelId=UCgiDRy6oyLanAcFeM4-_OYA&relatedToVideoId=eWXm5ZKGXSw&part=snippet,id&type=video&maxResults=10&key={your_api_key}`

And `https://www.googleapis.com/youtube/v3/search?relatedToVideoId=eWXm5ZKGXSw&part=snippet,id&type=video&maxResults=10&key={your_api_key}`

Will both return the same set of results.

Am I doing something wrong, or is this the intended behavior?

Answer: 
You're not doing anything wrong -- whether or not this is intended could only be answered by the engineering team, however. But it seems that the relatedToVideoId parameter is designed to ignore all other search filters (even 'q'). 

It seems logical that this is intended, as it is possibly tapping into the same algorithm that generates the related video thumbnails when a video is done playing (in other words, it's specifically used as a discovery tool for videos outside the keyword or channel relationships).
The above answer is correct, but if you still want to use this method and differentiate the channel's videos' to show yours only, you can, by doing this:
*(written in jQuery, but the same concept applies to other languages)*

```
var channelTitle = item.snippet.channelTitle;
var result = "";

if(channelTitle === "Your Channel Name")
{
    // print results
    $('.related-video').append(result);
    $(item).show(); // show item 
}
else // does not match channel name
{
    $(item).hide(); // hide item
}

```