Mongo queries to search all the collections of a database (Mongo/PyMongo) - mongodb

Question

I have been stuck on how to query db which the common data structure of every document looks as:
{
"_id": {
"$oid": "5e0983863bcf0dab51f2872b"
},
"word": "never", // get the `word` value for each of below queries
"wordset_id": "a42b50e85e",
"meanings": [{
"id": "1f1bca9d9f",
"def": "not ever",
"speech_part": "adverb",
"synonyms": ["ne'er"]
}, {
"id": "d35f973ed0",
"def": "not at all",
"speech_part": "adverb"
}]
}
1) query to get all the wordfor speech_part: "adverb" (eg: never,....) //
2)query to get all the word for: word length of 6 and speech_part: "adverb"
I have learnt from SO that ,to search whole collections first i have to retrieve all collections in the database , but how to write a query is where i stuck

Answer 1

db.collection.find({"meanings.speech_part":"adverb"},{"_id":0, "word":1})
To get array of all word of a specific speech_part above is the query.
First part of the query is filter predicate like in your scenario matching speach_part.if your matching column were not inside another object or a object inside a array, you could just write {column_name: "something"}.
as speech_part is inside an object which is inside an array, you have to write {"parentClumn.key":"something"}, in your case {"meanings.speech_part":"adverb"}.
where second part of the query is projection where you define which columns you want in your result. so to get only word column values you do {word:1}, to have more column you do {word:1, etc:1}. While mongodb project _id by default, so to remove _id from result you have to explicitly set {_id:0}
db.collection.find({
"meanings.speech_part":"adverb",
"$expr": { "$gt": [ { "$strLenCP": "$word" }, 6 ] }
},{"_id":0, "word":1})
To get array of all word of a specific speech_part with length greater than 6. This one is a bit complex query. You can look up $expr documentation. In $expr you can run function on your column and match the result. In your case strLenCP is calculating the length of your word column value and then checking, is it greater then 6 by $gt comparison operator

Answer 2

You may try below query to get the matching rows. You will have to try the same with pymongo.
db.getCollection('test-collection').find(
{
'meanings.speech_part': 'adverb'
},
{
_id: 0,
word: 1
}
);
Read about the projections in mongodb here:
https://docs.mongodb.com/manual/tutorial/project-fields-from-query-results