I am working with Django and I am a bit lost on how to extract information from models (tables).
I have a table containing different information from various sensors. What I would like to know is if it is possible from the Django models to obtain for each sensor (each sensor has an identifier) the last row of data (using the timestamp column).
In sql it would be something like this, (probably the query is not correct but I think you can understand what I'm trying)
SELECT sensorID,timestamp,sensorField1,sensorField2
FROM sensorTable
GROUP BY sensorID
ORDER BY max(timestamp);
I have seen that the group_by() function exists and also lastest() but I don't get anything coherent and I'm also not clear if I'm choosing the best form.
Can anyone help me get started with this topic? I imagine it is very easy but it is a new world and it is difficult to start.
Greetings!
When you use a PostgreSQL database, you can make use of the .distinct(..) method [Django-doc] of the queryset where you add fields that determine on what these should be distinct.
So you can obtain the latest sensors in Django with:
SensorModel.objects.order_by('sensor', '-timestamp').distinct('sensor')
We thus order by sensor (which is required for a .distinct(..)), and then in case of a tie (so two times the same sensor), we order on the timestamp in descending order, hence we pick the latest SensorModel object for that sensor.
Related
newly working with GraphQl and wondering is it possible to filter a set in a query? I'm still new to database design as well so could be an issue there.
So I run the below query, managerGwPicks has a field player with is a player object containig their name etc.
This player object contains a set of all the weeks they have played which is a separate table in my database.
So as can be seen in the above image when I display the set it shows all the gameweek data whereas ideally I would like it filtered by the gameweek:21 parameter passed to the managerGwPicks query.
I'm not sure it should be possible as there is no direct link between the managerGwPicks and playergwstats tables but I'd like to be sure that my thinking is correct.
My solution for my front end would be to have two queries, one similar to what I have getting the player information and a second query using the player id and gameweek to query playergwstats to get the player stats for the individual week. Does this sound like a reasonable approach?
It's preferable to avoid query patterns where you have to do multiple back-and-forths between the client and the server. If you can imagine making the link on the client then you can do it directly on the server. If you can go from a managerId and gameweek to a list of players and you can go from players to playergwstats then you can create a join that goes from your two parameters to all the relevant players.
I've covered patterns like this in a series of posts on GraphQL for SQL Developers - look at the join that's used to go from a booking reference to a series of tickets and flights.
I am trying to build a database for my website. There are currently three entries with different attributes in my database. I have not created these entries in order, but I have assigned a 'Chapter number' attribute which indicates the order 1,2,3.
I am now trying to inject this using 'context' and 'render' function in my views. I am using the method 'objects.all()' to add all objects to my context. I have a simple Html file where I am inserting the data from the database by looping over (a simple for loop) these added objects.
Now the output that is being generated (naturally) is that it is following the order in which I created the database. I am not sure how I can have the loop run in such a way that I get these chapters in correct order. Thank you for patiently reading my question. Any help will be appreciated.
You may use the order_by method which is included in Djangos QuerySet API:
https://docs.djangoproject.com/en/3.0/ref/models/querysets/
If you offer some more information of your specific data I might provide you with an example.
For orientation purposes, sorting queried objects by date would work as follows:
most_recent = Entry.objects.order_by('-timestamp')
You can sort by any field like so:
sorted_by_field = Entry.objects.order_by('custom_field')
In an InfluxDB measurement, how can the field values of points matching a query be updated? Is this still not easily doable as of v1.6?
As the example in that GitHub ticket suggested, what's the cleanest way of achieving something like this?
UPDATE access_log SET username='something' WHERE mac='xxx'
Anything better than driving it all from the client by updating individual points?
Q: How can the field values of points matching a query be updated? Is this still not easily doable as of v1.4?
A: From the best of my knowledge, there isn't an easy way to accomplish update in version 1.4 yet.
Field value of a point can only be updated by overriding. That is, to overwrite its value you'll need to know the details of your points. These details include its timestamp and series information, which is the measurement it reside and its corresponding tags.
Note: This "update" strategy can only be used for changing the field value but not tag value. To update a tag value you'll need to first DELETE the point data first and rewrite the entire point data with the updated tag and value.
Q: Anything better than driving it all from the client by updating individual points?
A: Influxdb supports multi-point write. So if you can build a filter to pre-select a small dataset of points, modify their field values and then override them in bulk.
Update is possible and would take the format:
INSERT measurement,tag_name=tag_value_no_quotes value_key_1=value_value_1,value_key_2=value_value_2 time
for example where I want to update the line with tag my_box at time 1526988768877018669 on the box measurement:
INSERT box,box_name=my_box item_1='apple',item_2='melon' 1526988768877018669
I am working on making an application in Django that can manage my GRE words and other details of each word. So whenever I add a new word to it that I have learnt, it should insert the word and its details in the database alphabetically. Also while retrieving, I want the details of the particular word I want to be extracted from the database.
Efficiency is the main issue.
Should I use SQLite? Should I use a file? Should I use a JSON object to store the data?
If using a file is the most efficient, what data structure should I implement?
Are there any functions in Django to efficiently do this?
Each word will have - meaning, sentence, picture, roots. How should I store all this information?
It's fine if the answer is not Django specific and talks about the algorithm or the type of database.
I'm going to answer from the data perspective since this is not totally related to django.
From your question it appears you have a fixed identifier for each "row": the word, which is a string, and a fixed set of attributes.
I would recommend using any enterprise level RDBMS. In the case of django, the most popular for the python ecosystem is PostgreSQL.
As for the ordering, just create the table with an index on the word name (this will be automatically done for you if you use the word as primary key), and retrieve your records using order_by in django.
Here's some info on django field options (check primary_key=True)
And here's the info for the order_by order_by method
Keep in mind you can also set the ordering in the Meta class of the model.
For your search case, you'll have to implement an endpoint that is capable of querying your database with startswith. You can check an example here
Example model:
class Word(models.Model):
word = models.CharField(max_length=255, primary_key=True)
roots = ...
picture = ...
On your second question: "Is this costly?"
It really depends. With 4000 words I'll say: NO
You probably want to add a delay in the client to do the query anyways (for example "if the user has typed in and 500ms have passed w/o further input")
If I'm to give 1 good advice to any starting developer, it's don't optimize prematurely
That seems simple enough, but all Django Queries seems to be 'SELECT *'
How do I build a query returning only a subset of fields ?
In Django 1.1 onwards, you can use defer('col1', 'col2') to exclude columns from the query, or only('col1', 'col2') to only get a specific set of columns. See the documentation.
values does something slightly different - it only gets the columns you specify, but it returns a list of dictionaries rather than a set of model instances.
Append a .values("column1", "column2", ...) to your query
The accepted answer advising defer and only which the docs discourage in most cases.
only use defer() when you cannot, at queryset load time, determine if you will need the extra fields or not. If you are frequently loading and using a particular subset of your data, the best choice you can make is to normalize your models and put the non-loaded data into a separate model (and database table). If the columns must stay in the one table for some reason, create a model with Meta.managed = False (see the managed attribute documentation) containing just the fields you normally need to load and use that where you might otherwise call defer(). This makes your code more explicit to the reader, is slightly faster and consumes a little less memory in the Python process.