问题
I have a book collection for example, each book has 'genre' field. How to query all books which genre is "fantasy" or "historical"? Like SQL SELECT * FROM book WHERE genre in ("fantasy", "historical"). Pretty usual SQL query as my opinion.
I found this feature request in GitHub, which isn't resolved yet. The Firestore documentation says: "Logical OR queries aren't supported". So, I cannot do such simple query in Firestore? What is a workaround? Should I query each genre separately and join the result?
Note, that I found similar questions like "How to get multiple documents by set of ids?", but not about custom properties. For ids, there is 'getAll()' method in the js admin sdk for example.
回答1:
It is not supported in firestore as you correctly identified. You can send two separate queries and merge the results locally. This is how it needs to be done unfortunately. If yours is a simple usecase, sending two separate requests shouldn't be a problem.
However, have a look at rxfire package which is officially supported by firebase. Note that it doesn't support or queries per se but with the help of rxjs, it makes such tasks easier to manage. Here's an excerpt from the link below on what it can do:
Firebase provides realtime streams and async callbacks to do all sorts of awesome things in your app. Getting data from one realtime stream is easy enough, but what if you want to join it from another? Combining multiple async streams of data can get complex. At Firebase, we wanted to help simplify things, so we created a new JavaScript library: RxFire.
Link to introductory blogpost: RxFire
来源：https://stackoverflow.com/questions/57181581/firestore-query-all-documents-which-field-matches-values-in-a-list-like-sql-i