Move subdocument to another document - mongodb

Question

I'm modelling a mongodb database that will have categories and articles.
The most common use will be listing articles through the categories, so I plan to make the articles as subdocuments of categories:
//db.categories.find()
[{
"id": 01,
"name": "Some category",
"articles: [{
"article_id": 01,
"title": "",
"content": ""
}]
}]
Sometimes, I might be moving articles from one category to another.
In a relational database, I should just update a foreign key, but in the above case, how can I move an article to another category?
Some details:
Preferable, the internal ObjectId should remain the same. But it's not mandatory.
Article might also contain subdocuments, if so, they must come along when moving.

Answer 1

You need first to get the article, then remove (pull) the article from its current category and then add (push) it into its new category, like this:
first get the article (article_id:01) from the category with id:01 ...
then pull the article with article_id:01 from the category with id:01
db.categories.update(
{"id" : 01, "articles.article_id" : 01}, // query
{"$pull" : {"articles" : {"article_id" : 01}}} // pull
)
and then push that same article (article_id:01) into its new category with id:02,
db.categories.update(
{"id" : 02}, // query
{"$push" : {"articles" : { // push
"article_id": 01,
"title": "",
"content": ""
}
}
}
)
if you want the array in the new category to be sorted after the push you have to use $each and $sort (see here).
For more info on $pull check here and for more info on $push check here.