How to query an array of documents in MongoDB - arrays

Question

I have an array of objects like this one:
{
"actions": [{
"help": {
"messages": [{}, {}]
}
}, {
"animals": [{
"sea": {
"messages": [{}, {}]
}
}, {
"land": {
"messages": [{}, {}]
}
}]
}]
}
I'm trying to get the messages array from each element. (Only the matched one)
I've tried something like:
db.getCollection('responses').find({"actions": "help"})
And
db.getCollection('responses').find({"actions.animals": "sea"})
With no luck as I'm still getting an empty array.
If this isn't possible, I'm open for alternatives.
I've been looking for similar questions like this one: Mongo db - Querying nested array and objects but in that question they're looking for a specific element inside the "messages" object (in my case) for example. Same as in this other question: Query for a field in an object in array with Mongo? where they're using $elementMatch and I don't think it fits my needs.
I thought that in the Mongo Tutorial: Query array of documents might help, but again, they have a similar structure like mine but they have a property for the help element for example, something like:
{
"actions": [{
"action": "help",
"messages": [{}, {}]
}, {
"action": "animals",
"classifications": [{
"classification": "sea",
"messages": [{}, {}]
}, {
"classification": "land",
"messages": [{}, {}]
}]
}]
}
Is this the only way to make it to work? Or can I still maintain my original structure?
Edit
After trying #Graciano's answer I'm getting the following:
db.getCollection('responses').aggregate([{
$project: { "messages": "$actions.animals.sea.messages" }
}])
Result:
/* 1 */
{
"_id" : ObjectId("5ac42e65734d1d5beda1f99b"),
"messages" : [
[
[
{
"type" : 0,
"speech" : "There are a lot of sea animals, to name a few: "
},
{
"type" : 0,
"speech" : "Whale\nFishes\nSharks"
}
]
]
]
}
Now the error to solve is that it must be a single array, not an array of arrays of arrays of messages... how to solve this?

Answer 1

I just updated your query and now it will look like this:
db.collection.aggregate([
{$project: { "messages": "$actions.animals.sea.messages" }},
{$unwind: "$messages"},
{$unwind: "$messages"}
])
And the result will be:
{
"_id" : ObjectId("5ac5b80dd39d9355012f6af3"),
"messages" : [
{
"type" : 0,
"speech" : "There are a lot of sea animals, to name a few: "
},
{
"type" : 0,
"speech" : "Whale\nFishes\nSharks"
}
]
}
Now you will get only single array, all you need to do $unwind the arrays respectively.

Answer 2

if all you need are the messages you can use an aggregation and create an Array from the elements you want
db.collection.aggregate([
{$project: { items: "$actions."+parameter+".messages" },
{$unwind: "$messages"},
{$unwind: "$messages"}
}])