@TMLA said:
How to get related movies in the same list?
I am making an app like Netflix which would recommend users with lists like "Popular Now" "Space Movies" etc.
I would personally create those lists and add movies and show it in the app.
But when the user press the movie to see the details I would also like to show the related movies.

There are several lists:

• User favorite list
• User recommendations list
• User ratings list
• User created list
• List of an API Discovery Request
  

I believe we have 2 possible resources for you:
https://developers.themoviedb.org/3/movies/get-movie-recommendations
https://developers.themoviedb.org/3/movies/get-similar-movies

You can make the basic API Request using the &append_to_response= Resource.
https://developers.themoviedb.org/3/movies/get-movie-details
https://developers.themoviedb.org/3/getting-started/append-to-response
Example
https://api.themoviedb.org/3/movie/550?api_key=THE_KEY&language=en-US&append_to_response=recommendations,similar

The problem is, I only want to show related movies that I have in my database.
So I can't use feature like 'keywords' etc. I need to show related movies only form what I have.
How do I acheive this?

As for filtering by only the IDs that are already in your database,
I believe that there is only one solution on your side.
I don't think it's possible to filter on &append_to_response=
with a parameter like ,similar_only_in_my_DB and/or ,recommendations_only_in_my_DB.

As your question is related to API I will move to API Support.