I can do
db.find({'node_type': {'$regex':'^meta-'}}).sort("id", pymongo.DESCENDING)
I assume find returns a lazy iterator and sort modifies it in-place, and no data is fetched unless the iterator is used.
But I want to use find_one with sort parameters; in this case, fetch the document with the highest ID. What's the correct usage for this need?
You cannot use find_one with sorting, because find_one returns document directly (or None). So you can use find with sort or you can use the aggregation framework:
db.collection.aggregate([
{$match: {node_type: {$regex: '^meta-'}}},
{$sort: {id: -1}},
{$limit: 1}
]);
You can use a simple .limit(1)
db.find({'node_type': {'$regex':'^meta-'}}).limit(1).sort("_id", -1)
or
db.find({'node_type': {'$regex':'^meta-'}}, limit=1).sort("_id", -1)
Related
I am trying to create an index that returns entire data object of the documents in a collection;
here is the code:
CreateIndex({
name: "users_by_data",
source: Collection("users"),
values: { field: ['data'] }
})
but after creation it says: Values Not set (using ref by default)
If I specifically define fields (separately by their name), it will behave as expected, but data isn't working. the question is:
Is it impossible (e.g. for performance reasons) or am I doing it wrong?
side note: I am aware that I can do Lambda function on Paginate and achieve similar result, but this question specifically is about the Index level;
You can currently index regular values (strings, numbers, dates, etc) and you can index an array which will more or less 'unroll' the array into separate index entries. However, what you are trying, indexing an object is not possible at this point. An object (like data) will be ignored if you try to index it.
Currently, you have two options:
as you mentioned, using Map/Get at query time.
listing all values of the data object in the index since you can select specific values of an object in the index (which is however less flexible if new attributes arrive later on in the object)
We intend to support indexing of objects in the future, I can't provide an ETA yet though. There is a feature request on our forums as well that you can vote up: https://forums.fauna.com/t/object-as-terms-instead-of-scalar-s/628
You're going to want to use the Select function on the Ref you get back from the Index if you only want the data field back.
For an individual document, you can do something like this
Select( "data",
Get(
Match(
Index("yourIndexName"),
**yourIndexTerm // Could point to String/Number/FQL Ref
)
)
)
For a list of documents, you can use Paginate as you said but you can still pull the data property out of each document
Map(
Paginate(
Match(
Index("yourIndexName"),
**yourIndexTerm // Could point to String/Number/FQL Ref
)
),
Lambda("doc", Select("data", Get(Var("doc"))))
)
I'm new to Aerospike and am probably missing something fundamental, but I'm trying to see an enumeration of the Keys in a Set (I'm purposefully avoiding the word "list" because it's a datatype).
For example,
To see all the Namespaces, the docs say to use SHOW NAMESPACES
To see all the Sets, we can use SHOW SETS
If I want to see all the unique Keys in a Set ... what command can I use?
It seems like one can use client.scan() ... but that seems like a super heavy way to get just the key (since it fetches all the bin data as well).
Any recommendations are appreciated! As of right now, I'm thinking of inserting (deleting) into (from) a meta-record.
Thank you #pgupta for pointing me in the right direction.
This actually has two parts:
In order to retrieve original keys from the server, one must -- during put() calls -- set policy to save the key value server-side (otherwise, it seems only a digest/hash is stored?).
Here's an example in Python:
aerospike_client.put(key, {'bin': 'value'}, policy={'key': aerospike.POLICY_KEY_SEND})
Then (modified Aerospike's own documentation), you perform a scan and set the policy to not return the bin data. From this, you can extract the keys:
Example:
keys = []
scan = client.scan('namespace', 'set')
scan_opts = { 'concurrent': True, 'nobins': True, 'priority': aerospike.SCAN_PRIORITY_MEDIUM }
for x in (scan.results(policy=scan_opts)): keys.append(x[0][2])
The need to iterate over the result still seems a little clunky to me; I still think that using a 'master-key' Record to store a list of all the other keys will be more performant, in my case -- in this way, I can simply make one get() call to the Aerospike server to retrieve the list.
You can choose not bring the data back by setting includeBinData in ScanPolicy to false.
I see that performing a .first() or .last() query on an ActiveRecord query returns an array instead of an ActiveRecord::AssociationRelation. This prevents me from appending an .order() clause as that is not an array method.
Stepping back to the goal for a minute, I want to pull the latest five ("live", ie. active) comments related to a post and this is the code that fails:
#post.comments.where(status: "live").last(5).order(id: :desc)
The error is
undefined method `order' for #<Array:0x00000011b096c0>
If I remove the .last() clause, or the .order() clause I get a valid result but obviously not the result set that I am angling for.
What's the best way to get the last five live comments in reverse chronological order? More importantly, perhaps, where is the best documentation / tutorial on this syntax? I find the official guide too brief.
Talking about documentation, I could recommend Rails API. It has a lot of detailed descriptions for each interface method.
What about your question, you could use limit method instead of first or last:
#post.comments.where(status: "live").limit(5).order(id: :desc)
It does not affect the type of result (it stays ActiveRecord::AssociationRelation). Hence, if no result is found, this relation with to_a method will be converted to an empty array. This is different from the behavior of first and last methods which return nil in case of no result.
How about something like:
#post.comments.where(status: 'live').order(created_at: :desc).take(5)
I just plugged in created_at. Maybe you'll want updated_at instead.
I have a model:
class List:
data = ...
previous = models.ForeignKey('List', related_name='r1')
obj = models.ForeignKey('Obj', related_name='nodes')
This is one direction list containing reference to some obj of Obj class. I can reverse relation and get some list's all elements refering to obj by:
obj.nodes
But how Can I get the very last node? Without using raw sql, genering as little SQL queries by django as can.
obj.nodes is a RelatedManager, not a list. As with any manager, you can get the last queried element by
obj.nodes.all().reverse()[0]
This makes sense anyway only if there is any default order defined on the Node's Meta class, because otherwise the semantic of 'reverse' don't make any sense. If you don't have any specified order, set it explicitly:
obj.nodes.order_by('-pk')[0]
len(obj.nodes)-1
should give you the index of the last element (counting from 0) of your list
so something like
obj.nodes[len(obj.nodes)-1]
should give the last element of the list
i'm not sure it's good for your case, just give it a try :)
I see this question is quite old, but in newer versions of Django there are first() and last() methods on querysets now.
Well, you just can use [-1] index and it will return last element from the list. Maybe this question are close to yours:
Getting the last element of a list in Python
for further reading, Django does not support negative indexing and using something like
obj.nodes.all()[-1]
will raise an error.
in newer versions of Django you can use last() function on queryset to get the last item of your list.
obj.nodes.last()
another approach is to use len() function to get the index of last item of a list
obj.nodes[len(obj.nodes)-1]
Specifically, I have a model that has a field like this
pub_date = models.DateField("date published")
I want to be able to easily grab the object with the most recent pub_date. What is the easiest/best way to do this?
Would something like the following do what I want?
Edition.objects.order_by('pub_date')[:-1]
obj = Edition.objects.latest('pub_date')
You can also simplify things by putting get_latest_by in the model's Meta, then you'll be able to do
obj = Edition.objects.latest()
See the docs for more info. You'll probably also want to set the ordering Meta option.
Harley's answer is the way to go for the case where you want the latest according to some ordering criteria for particular Models, as you do, but the general solution is to reverse the ordering and retrieve the first item:
Edition.objects.order_by('-pub_date')[0]
Note:
Normal python lists accept negative indexes, which signify an offset from the end of the list, rather than the beginning like a positive number. However, QuerySet objects will raise AssertionError: Negative indexing is not supported. if you use a negative index, which is why you have to do what insin said: reverse the ordering and grab the 0th element.
Be careful of using
Edition.objects.order_by('-pub_date')[0]
as you might be indexing an empty QuerySet. I'm not sure what the correct Pythonic approach is, but the simplest would be to wrap it in an if/else or try/catch:
try:
last = Edition.objects.order_by('-pub_date')[0]
except IndexError:
# Didn't find anything...
But, as #Harley said, when you're ordering by date, latest() is the djangonic way to do it.
This has already been answered, but for more reference, this is what Django Book has to say about Slicing Data on QuerySets:
Note that negative slicing is not supported:
>>> Publisher.objects.order_by('name')[-1]
Traceback (most recent call last):
...
AssertionError: Negative indexing is not supported.
This is easy to get around, though. Just change the order_by()
statement, like this:
>>> Publisher.objects.order_by('-name')[0]
Refer the link for more such details. Hope that helps!