Qt version: 5.8
Let's say I have the following SQL tables
-- People
person_id | first_name | last_name | age
-- Cars, person_id is a foreign key to show that this person owns this car
car_id | car_year | car_make | car_model | person_id
Let's say I want to populate the following Table View or Table Widget with a mixture of that data like so
// Table that the user sees. Notice that not all the information from the tables is shown.
first_name | last_name | car_year | car_make | car_model
What is the best/recommended way to do this? I can see the following two ways, but I feel neither are the best way to do this
Use a Table Widget, which is an item-based table view with a default model. To do this, I'm guessing I would need to make QSqlQuerys to get the data from my QSqlDatabase and just populate the Table Widget that way.
Use a Table View, which would require me to create my own QSqlTableModel for the data model of the view. According to the documentation for QSqlTableModel, it is a high-level interface for reading and writing database records from a single table. This means I would need two QSqlTableModels, one for each of my tables above. However, the Table View can only use one model, and it will show all the data from that model. I think the only way this would work is to combine the tables into one table with only the information I want the user to see. I feel like that would be very ugly but possible. In that case, should I have three tables total - the two above plus the combined one for the users to see?
I feel like #1 is the better of those two, but I'm wondering if there's still a better way than both of those.
If person_id is primary key of table people you can use QtSql.QsqlRelationalTableModel to show data from several tables in an QtWidgets.QTableView, here your example:
QSqlRelationalTableModel rm = new QSqlRelationalTableModel(parentObject, database);
rm→setTable(„cars“);
rm→setRelation(4, QSqlRelation(„people“, „person_id“, „first_name, last_name“);
rm→select();
QTableView tv = new QTableView();
tv→setModel(rm);
tv→hideColumn(0); # hide column car_id
hh = tv->horizontalHeader();
hh→moveSection(4, 0); # change order of columns
hh→moveSection(5, 1);
Related
I am working on converting some relatively complex SQL into something that Django can play with. I am trying not to just use the raw SQL, since I think playing with the standard Django toolkit will help me learn more about Django.
I have already managed to break up parts of the sql into chunks, and am tackling them piecemeal to make things a little easier.
Here is the SQL in question:
SELECT i.year, i.brand, i.desc, i.colour, i.size, i.mpn, i.url,
COALESCE(DATE_FORMAT(i_eta.eta, '%M %Y'),'Unknown')
as eta
FROM i
JOIN i_eta ON i_eta.mpn = i.mpn
WHERE category LIKE 'kids'
ORDER BY i.brand, i.desc, i.colour, FIELD(size, 'xxl','xl','l','ml','m','s','xs','xxs') DESC, size+0, size
Here is what I have (trying to convert line by line):
(grabbed automatically when performing filters)
(have to figure out django doc on coalesce for syntax)
db alias haven't been able to find yet - it is crucial since there is a db view that requires it
already included in the original q
.select_related?
.filter(category="kids")
.objects.order_by('brand','desc','colour') - don't know how to deal with SQL FIELDS
Any advice would be appreciated!
Here's how I would structure this.
First, I'm assuming your models for i and i_eta look something like this:
class I(models.Model):
mpn = models.CharField(max_length=30, primary_key=True)
year = models.CharField(max_length=30)
brand = models.CharField(max_length=30)
desc = models.CharField(max_length=100)
colour = models.CharField(max_length=30)
size = models.CharField(max_length=3)
class IEta(models.Model):
i = models.ForeignKey(I, on_delete=models.CASCADE)
eta = models.DateField()
General thoughts:
To write the coalesce in Django: I would not replace nulls with "Unknown" in the ORM. This is a presentation-layer concern: it should be dealt with in a template.
For date formatting, you can do date formatting in Python.
Not sure what a DB alias is.
For using multiple tables together, you can use either select_related(), prefetch_related(), or do nothing.
select_related() will perform a join.
prefect_related() will get the foreign key ID's from the first queryset, then generate a query like SELECT * FROM table WHERE id in (12, 13, 14).
Doing nothing will work automatically, but has the disadvantage of the SELECT N+1 problem.
I generally prefer prefetch_related().
For customizing the sort order of the size field, you have three options. My preference would be option 1, but any of the three will work.
Denormalize the sort criteria. Add a new field called size_numeric. Override the save() method to populate this field when saving new instances, giving xxl the value 1, xl the value 2, etc.
Sort in Python. Essentially, you use Python's built-in sorting methods to do the sort, rather than sorting it in the database. This doesn't work well if you have thousands of results.
Invoke the MySQL function. Essentially, using annotate(), you add the output of a function to the queryset. order_by() can sort by that function.
I'm new to django and ORM in general, and so have trouble coming up with query which would join multiple tables.
I have 4 Models that need joining - Category, SubCategory, Product and Packaging, example values would be:
Category: 'male'
SubCategory: 'shoes'
Product: 'nikeXYZ'
Packaging: 'size_36: 1'
Each of the Model have FK to the model above (ie. SubCategory has field category etc).
My question is - how can I filter Product given a Category (e.g. male) and only show products which have Packaging attribute available set to True? Obviously I want to minimise the hits on my database (ideally do it with 1 SQL query).
I could do something along these lines:
available = Product.objects.filter(packaging__available=True)
subcategories = SubCategory.objects.filter(category_id=<id_of_male>)
products = available.filter(subcategory_id__in=subcategories)
but then that requires 2 hits on database at least (available, subcategories) I think. Is there a way to do it in one go?
try this:
lookup = {'packaging_available': True, 'subcategory__category_id__in': ['ids of males']}
product_objs = Product.objects.filter(**lookup)
Try to read:
this
You can query with _set, multi __ (to link models by FK) or create list ids
I think this should work but it's not tested:
Product.objects.filter(packaging__available=True,subcategories__category_id__in=[id_of_male])
it isn't tested but I think that subcategories should be plural (related_name), if you didn't set related_name, then subcategory__set instead od subcategories should work.
Probably subcategories__category_id__in=[id_of_male] can be switched to .._id=id_of_male.
A relevant image of my model is here: http://i.stack.imgur.com/xzsVU.png
I need to make a queryset that contains all cats who have an associated person with a role of "owner" and a name of "bob".
The sql for this would be shown below.
select * from cat where exists
(select 1 from person inner join role where
person.name="bob" and role.name="owner");
This problem can be solved in two sql queries with the following django filters.
people = Person.objects.filter(name="bob", role__name="owner")
ids = [p.id for p in people]
cats = Cat.objects.filter(id__in=ids)
My actual setup is more complex than this and is dealing with a large dataset. Is there a way to do this with one query? If it is impossible, what is the efficient alternative?
I'm pretty sure this is your query:
cats = Cat.objects.filter(person__name='bob', person__role__name='owner')
read here about look ups spanning relationships
I have a group of related companies that share items they own with one-another. Each item has a company that owns it and a company that has possession of it. Obviously, the company that owns the item can also have possession of it. Also, companies sometimes permanently transfer ownership of items instead of just lending it, so I have to allow for that as well.
I'm trying to decide how to model ownership and possession of the items. I have a Company table and an Item table.
Here are the options as I see them:
Inventory table with entries for each Item - Company relationship. Has a company field pointing to a Company and has Boolean fields is_owner and has_possession.
Inventory table with entries for each Item. Has an owner_company field and a possessing_company field that each point to a Company.
Two separate tables: ItemOwner and ItemHolder**.
So far I'm leaning towards option three, but the tables are so similar it feels like duplication. Option two would have only one row per item (cleaner than option one in this regard), but having two fields on one table that both reference the Company table doesn't smell right (and it's messy to draw in an ER diagram!).
Database design is not my specialty (I've mostly used non-relational databases), so I don't know what the best practice would be in this situation. Additionally, I'm brand new to Python and Django, so there might be an obvious idiom or pattern I'm missing out on.
What is the best way to model this without Company and Item being polluted by knowledge of ownership and possession? Or am I missing the point by wanting to keep my models so segregated? What is the Pythonic way?
Update
I've realized I'm focusing too much on database design. Would it be wise to just write good OO code and let Django's ORM do it's thing?
Is there a reason why you don't want your item to contain the relationship information? It feels like the owner and possessor are attributes of the item.
class Company(models.Model):
pass
class Item(models.Model):
...
owner = models.ForeignKey(Company, related_name='owned_items')
holder = models.ForeignKey(Company, related_name='held_items')
Some examples:
company_a = Company.objects.get(pk=1)
company_a.owned_items.all()
company_a.held_items.all()
items_owned_and_held_by_a=Items.objects.filter(owner=company_a, holder=company_a)
items_on_loan_by_a=Items.objects.filter(owner=company_a).exclude(holder=company_a)
#or
items_on_loan_by_a=company_a.owned_items.exclude(holder=company_a)
items_a_is_borrowing=Items.objects.exclude(owner=company_a).filter(holder=company_a)
#or
items_a_is_borrowing=company_a.held_items.exclude(owner=company_a)
company_b = Company.objects.get(pk=2)
items_owned_by_a_held_by_b=Items.objects.filter(owner=company_a, holder=company_b)
#or
items_owned_by_a_held_by_b=company_a.owned_items.filter(holder=company_b)
#or
items_owned_by_a_held_by_b=company_b.held_items.filter(owner=company_a)
I think if your items are only owned by a single company and held by a single company, a separate table shouldn't be needed. If the items can have multiple ownership or multiple holders, a m2m table through an inventory table would make more sense.
class Inventory(models.Model):
REL = (('O','Owns'),('P','Possesses'))
item = models.ForeignKey(Item)
company = models.ForeignKey(Company)
relation = models.CharField(max_length=1,choices=REL)
Could be one implementation, instead of using booleans. So I'd go for the first. This could even serve as an intermediate table if you ever decide to use a 'through' to relate items to company like this:
Company:
items = models.ManyToManyField(Item, through=Inventory)
Option #1 is probably the cleanest choice. An Item has only one owner company and is possessed by only one possessing company.
Put two FK to Company in Item, and remember to explicitly define the related_name of the two inverses to be different each other.
As you want to avoid touching the Item model, either add the FKs from outside, like in field.contribute_to_class(), or put a new model with a one-to-one rel to Item, plus the foreign keys.
The second method is easier to implement but the first will be more natural to use once implemented.
I have an application that allows for "contacts" to be made completely customized. My method of doing that is letting the administrator setup all of the fields allowed for the contact. My database is as follows:
Contacts
id
active
lastactive
created_on
Fields
id
label
FieldValues
id
fieldid
contactid
response
So the contact table only tells whether they are active and their identifier; the fields tables only holds the label of the field and identifier, and the fieldvalues table is what actually holds the data for contacts (name, address, etc.)
So this setup has worked just fine for me up until now. The client would like to be able to pull a cumulative report, but say state of all the contacts in a certain city. Effectively the data would have to look like the following
California (from fields table)
Costa Mesa - (from fields table) 5 - (counted in fieldvalues table)
Newport 2
Connecticut
Wallingford 2
Clinton 2
Berlin 5
The state field might be id 6 and the city field might be id 4. I don't know if I have just been looking at this code way to long to figure it out or what,
The SQL to create those three tables can be found at https://s3.amazonaws.com/davejlong/Contact.sql
You've got an Entity Attribute Value (EAV) model. Use the field and fieldvalue tables for searching only - the WHERE caluse. Then make life easier by keeping the full entity's data in a CLOB off the main table (e.g. Contacts.data) in a serialized format (WDDX is good for this). Read the data column out, deserialize, and work with on the server side. This is much easier than the myriad of joins you'd need to do otherwise to reproduce the fully hydrated entity from an EAV setup.