The following document records a conversation between Milhouse and Bart. I would like to insert a new message with the right num (the next in the example would be 3) in a unique operation. Is that possible ?
{ user_a:"Bart",
user_b:"Milhouse",
conversation:{
last_msg:2,
messages:[
{ from:"Bart",
msg:"Hello"
num:1
},
{ from:"Milhouse",
msg:"Wanna go out ?"
num:2
}
]
}
}
In MongoDB, arrays keep their order, so by adding a num attribute, you're only creating more data for something that you could accomplish without the additional field. Just use the position in the array to accomplish the same thing. Grabbing the X message in an array will provide faster searches than searching for { num: X }.
To keep the order, I don't think there's an easy way to add the num category besides does a find() on conversation.last_msg before you insert the new subdocument and increment last_msg.
Depending on what you need to keep the ordering for, you might consider including a time stamp in your subdocument, which is commonly kept in conversation records anyway and may provide other useful information.
Also, I haven't used it, but there's a Mongoose plugin that may or may not be able to do what you want: https://npmjs.org/package/mongoose-auto-increment
You can't create an auto increment field but you can use functions to generate and administrate sequence :
http://docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/
I would recommend using a timestamp rather than a numerical value. By using a timestamp, you can keep the ordering of the subdocument and make other use of it.
Related
my database like;
I want, when announcement0 field is deleted, announcement1 field name to change announcement0. Is there a way to do this ?
There is no way to rename fields in Firestore, let alone to have that happen automatically.
It sounds like you have multiple announcements in your document however. In that case, you could consider storing all announcements in a single array field announcements. In an array field, when you remove the first item (at index 0) all other items after that shift down in the array to take its place, which seems to be precisely what you want.
You cannot rename fields in a document. You'll have to delete and recreate it.
Now I'm assuming the number just defines the order of document. If that's the case you can use this workaround, instead of looking for 'announcement0' on client side, you can just store a number field in the document such as 0 in announcement0 and so on. So to get announcement1 when announcement0 is deleted you can uses this query:
const firstAnnouncement = await dbRef.orderBy('number').limit(1).get()
This will get the announcement with least number (highest rank). You can change the limit as per your needs.
But if renaming fields is needed then you'll have to delete and recreate all trailing announcements.
I'm new to monogDB and trying to design the way I store my data so I can do the kinds of queries I want to. Say I have a document that looks like
{
"foo":["foo1","foo2","foo3"],
"bar":"baz"
}
Where the array "foo" is always of length 3, and the order of the items are meaningful. I would like to be able to make a query that searches for all documents where "foo2" == something. Essentially I want to treat "foo" like any old array and be able to index it in a search, so something like "foo"[1] == something.
Does monogDB support this? Would it be more correct to store my data like,
{
"foo":{
"foo1":"val1",
"foo2":"val2",
"foo3":"val3"
},
"bar":"baz"
}
instead? Thanks.
The schema you have asked about is fine.
To insert at a specific index of array:
Use the $position operator. Read here.
To query at a specific index location:
Use the syntax key.index. As in:
db.users.find({"foo.1":"foo2"})
I have a mongo collection with documents that have a schema structured like the following:
{ _id : bla,
fname : foo,
lname : bar,
subdocs [ { subdocname : doc1
field1 : one
field2 : two
potentially_huge_array : [...]
}, ...
]
}
I'm using the ruby mongo driver that currently does not support elemMatch. I use an aggregation when extracting from subdocs via a project, unwind and match pipeline.
What I would now like to do is to page results from the potentially_huge_array array contained in the subdocument. I have not been able to figure out how to grab just a subset of the array without dragging the entire subdoc, huge array and all, out of the db into my app.
Is there some way to do this?
Would a different schema be a better way to handle this?
Depending on how huge is huge, you definitely don't want it embedded into another document.
The main reason is that unless you always want the array returned with the document, you probably don't want to store it as part of the document. How you can store it in another collection would depend on exactly how you want to access it.
Reviewing the types of queries you most often perform on your data will usually suggest the best schema - one that will allow you to be efficient about number of queries, the amount of data returned and ease of indexing the data.
If you field really huge and changes often, just placed it in separate collection.
I have a table called devicesegments, each row of which contains a large array called devices. Owing to the size of the device array, I've been asked not to include it in my query for a page that lists all devicesegments, but only include their count. Is this possible?
What I was doing before :
A simple db.devicesegments.find()
What I'm doing now :
db.devicesegments.find({}, { devices : 0 })
What I want to achieve :
db.devicesegments.find({}, { devices : 0, devices.length : 1 })
Something like a COUNT(devices) AS device_count!
Ashkay, there's no way to do this with Mongo currently. As #rompetroll says, your application should keep a "count" field on each document, and carefully $inc it whenever you change the number of entries in the array. Then when you query for the document, exclude the array like:
db.collection.find({}, {devices:0})
If you're willing to run MongoDB 2.1, which is a development release, the aggregation framework provides a means to calculate the array size within a query:
http://www.mongodb.org/display/DOCS/Aggregation+Framework
Since there is no way to currently do this, without including a new device_count in my table, the temporary fix that I applied was to fetch all the data from the database, along with the devices array, and for each row, add a field for devices.length and then remove the devices array before sending the data across.
I want to retrieve the last row from the data store so how can i do that??
I know the long method i.e
for(Table_name e: resultset)
{
cnt++;
}
results.get(cnt).getvalue();
I have String with (number) as primary key.can i use it to get descending order???
Is there any method through which i can get the last row???
You should probably sort in the opposite order (if possible for your query, the data store has some restrictions here) and get the first result of that.
Also, if you store numbers in String fields the order may not be what you want it to be (you might need padding here).