I am working on a database for an ordered tasklist. As some of the tasks are repetitive I need a "taskList" that can again consist of tasks and other task lists.
Find example here
In the above example TaskList 1 consists of [TaskList2, Task5, Task 6, TaskList3], where TaskList2 consists of [Task1, Task2, Task3, Task4] and so on.
My main question is:
What's the best way to set up a Database-Schema to handle this nested polymorphism? (I am using Laravels ORM)
Conceptually, your problem is solved by considering a tasklist as an ordered aggregation of tasklist items, which are tasks or (other) tasklists, as described by the following class diagram:
A DB schema that implements this conceptual model is provided by the following table schemas (where PK/FK = primary/foreign keys, and ? = optional):
tasklists( PK id, name)
tasks( PK id, name)
tasklistitems(
FK tasklist REF tasklists,
seq_no,
FK taskitem? REF tasks,
FK tasklistitem? REF tasklists
)
such that each row of tasklistitems is related to a tasklist via its first (foreign key) attribute and references either a task or a tasklist via the corresponding optional foreign key attributes taskitem and tasklistitem.
Notice that the sequence number attribute of tasklist items implements their ordering.
Related
I've a model which is just a relation between two entities like this:
class SomeModel(models.Model):
a = OneToOneField(User,primary_key=True,...)
b = ForeignKey(Car,...)
As per design, it was correct, as I didn't want an User to have multiple Car. But in my new design, I want to accept multiple Car to an User. So I was trying something like this:
class SomeModel(models.Model):
class Meta:
unique_together = (("a", "b"),)
a = ForeignKey(User,...)
b = ForeignKey(Car,...)
But during migration, it asks me:
You are trying to add a non-nullable field 'id' to somenmodel without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
2) Quit, and let me add a default in models.py
Select an option:
I just wanted to remove that foreign key from OneToOneRelation and add a new id combining both. - How to resolve this?
Delete the model and table from the database and create a new model from starting or add null=True to the given field. you must be having some data (rows) in your table and now you are creating a new column, so previous rows need something to be there in the newly created column, writing null = True will do that.
I'm very much a beginner when it comes to database relationships hence what I suspect is a basic question! I have two database tables as follows:
Projects
id
company_id
name
etc...
rfis
id
project_id (foreign key is id on the Projects table above)
Number (this is the column I need help with - more below)
question
The relationships at the Model level for these tables are as follows:
Project
public function rfi()
{
return $this->hasMany('App\Rfi');
}
RFI
public function project()
{
return $this->belongsTo('App\Project');
}
What I'm trying to achieve
In the RFI table I need a system generated number or essentially a count of RFI's. Where I'm finding the difficulty is that I need the RFI number/count to start again for each project. To clarify, please see the RFI table below which I have manually created with the the 'number' how I would like it displayed (notice it resets for each new project and the count starts from there).
Any assistance would be much appreciated!
Todd
So the number field depends on the number of project_id in the RFI table. It is exactly the number of rows with project_id plus one.
So when you want to insert a new row, you calculate number based on project_id and assign it.
RFI::create([
'project_id' => $project_id,
'number' => RFI::where('project_id', $project_id)->count() + 1,
...
]);
What I understood is that you want to set the value of the "number" field to "1" if it's a new project and "increment" if it's an existing project. And you want to automate this without checking for it every time you save a new row for "RFI" table.
What you need is a mutator. It's basically a method that you will write inside the desired Model class and there you will write your own logic for saving data. Laravel will run that function automatically every time you save something. Here you will learn more about mutators.
Use this method inside the "RFI" model class.
public function setNumberAttribute($value)
{
if(this is new project)
$this->attributes['number'] = 1;
else
$this->attributes['number']++;
}
Bonus topic: while talking about mutators, there's also another type of method called accessor. It does the same thing as mutators do, but just the opposite. Mutators get called while saving data, accessors get called while fetching data.
When one models resource relationships, the classic example give is for articles and comments. This becomes simple to model because:
articles can be uniquely identified: /articles/1
comments can be uniquely identified: /comments/2
their relationship can be provided as /articles/1/comments which would return only comments for that article
But how does one handle the case where the related resource only exists within the context of the parent?
For example, order and items:
order can be uniquely identified /orders/123
the items in that order only exist as /orders/123/items
and NOT as /items
In JSON-API the relationship object needs a "type" and "id" to specify the linkage:
"relationships": {
"links": {
"self": "http://example.com/orders/123/relationships/items",
"related": "http://example.com/orders/123/items"
},
"data": {
"type": <what goes here>,
"id": <what goes here>
}
}
The type and id for data would need to be relative to order number 123. Assuming, of course, that they don't get assigned a UUID or similar from a database because they are, in fact, a composite key. They exist primarily as a foreign key combination.
How does one handle that?
One option for the relation is to use type as order_item and id as a hash or something delimited string concatenation of the order's id and the item's id. (e.g. 123_abc). 123 I get from the order and abc I get from the item in the order.
Is there another way other than avoiding the need altogether to supply resource linkage?
Every resource must be uniquely identified by a combination of type and id according to JSON API specification:
Identification
Every resource object MUST contain an id member and a type member. The values of the id and type members MUST be strings.
Within a given API, each resource object’s type and id pair MUST identify a single, unique resource. (The set of URIs controlled by a server, or multiple servers acting as one, constitute an API.)
https://jsonapi.org/format/#document-resource-object-identification
Therefor you are not only requiring an ID for resource linkage but also to build any valid response including such a resource.
But there are no rules about how you generate that unique IDs per type. Combining an unique order ID with an unique item ID to get an unique ID per item in an order seems to be a fine approach if your data model doesn't include an ID.
I have models:
class Reference(models.Model):
name = models.CharField(max_length=50)
class Search(models.Model):
reference = models.ForeignKey(Reference)
update_time = models.DateTimeField(auto_now_add=True)
I have an instance of Reference and i need to get all last searches for the reference. Now i am doing it in this way:
record = Search.objects.filter(reference=reference)\
.aggregate(max_date=Max('update_time'))
if record:
update_time = record['max_date']
searches = reference.search_set.filter(update_time=self.update_time)
It is not a big deal to use 2 queries except the one but what if i need to get last searches for each reference on a page? I would have got 2x(count of references) queries and it would not be good.
I was trying to use this solution https://stackoverflow.com/a/9838438/293962 but it didn't work with filter by reference
You probably want to use the latest method.
From the docs, "Returns the latest object in the table, by date, using the field_name provided as the date field."
https://docs.djangoproject.com/en/1.8/ref/models/querysets/#latest
so your query would be
Search.objects.filter(reference=reference).latest('update_time')
I implemented a snippet from someone in gist but I don't remember the user neither have the link.
A bit of context:
I have a model named Medicion that contains the register of mensuration of a machine, machines are created in a model instance of Equipo, Medicion instances have besides of a Foreign key to Equipo, a foreign key to Odometro, this model serves as a kind of clock or metre, that's why when I want to retrieve data (measurements aka instances of Medicion model) for a certain machine, I need to indicate the clock as well, otherwise it would retrieve me a lot of messy and unreadable data.
Here is my implementation:
First I retrieve the last dates:
ult_fechas_reg = Medicion.objects.values('odometro').annotate(max_fecha=Max('fecha')).order_by()
Then I instance an Q object:
mega_statement = Q() # This works as 'AND' Sql Statement
Then looping in every date retrieved in the queryset(annotation) and establishing the Q statement:
for r in ult_fechas_reg:
mega_statement |= (Q(odometro__exact=r['odometro']) & Q(fecha=r['max_fecha']))
Finally passed this mega statement to the queryset that pursues to retrieve the last record of a model filtered by two fields:
resultados = Medicion.objects.filter(mega_query).filter(
equipo=equipo,
odometro__in=lista_odometros).order_by('odometro', 'fecha') # lista_odometros is a python list containing pks of another model, don't worry about it.
[Edit: Apparently, this is only an issue for arrays and FoxyBOA's answer might direct to (or even is) the answer.]
My question relates to these software: Hibernate3+Annotation, Spring MVC, MySQL and in this example also Spring Security.
I was wondering, why collections, which are automatically associated by Hibernate contain null values for each row number of the child table (besides the elements which are correct). My Example:
I have a users and an authorities table, the primary key of the users table is username which serves as foreign key. Right now, there are 13 rows in my authorities table. When I retrieve a user from the database (MySQL InnoDB) and Hibernate automatically retrieves the user's authorities corresponding to this mapping:
#OneToMany
#JoinColumn(name = "username")
#IndexColumn(name="id") // "id" was the primary key and is used to sort the elements
public Authority[] getAuthorities() {
return authorities;
}
public void setAuthorities(Authority[] authorities) {
this.authorities = authorities;
}
... I end up with a collection "authorities" containing 14 (0-13) elements of which only four are not-null (four rows in the database table belong to that specific user, so that is correct). As far as I realize, I am using Hibernate defaults for properties like Fetchmode etc. I am getting the user like this:
Criteria criteria = getSession().createCriteria(User.class);
criteria.add(Restrictions.eq("username",username));
User user = (User) criteria.uniqueResult();
The logging information from org.hibernate.loader.loader correctly "mentions" four rows for the resultset. Still, the user created has the four correct elements plus ten null values in the Array. In my specific example, this results in this exception:
java.lang.IllegalArgumentException: Granted authority element 0 is null - GrantedAuthority[] cannot contain any null elements
The answer lies in the #IndexColumn annotation. It is using the value of id as the array index, thus the number of elements in the Array is basically going to be the value of the highest ID in the Authorities table.
see the hibernate documentation on indexed collections
try removing the annotation.
Also just as a thought; have you considered using a Set for the mapping? it isn't strictly necessary, it just a bit more common form of mapping that's all.
I can recommend you check your data. If you have a missed indexes (id column in your case), then instead of missed id you'll get null in your array.
I.e.
table authorities:
username id
bob 1
bob 3
bob 5
As a result you will have an array:
{0=null, 1=bob, 2=null, 3=bob, 4=null, 5=bob}
UPDATE:
I met the situation in two cases:
Missed key values in indexed column id at authorities table (e.g. 0,1,3,4,5 - missing value 2. Hibernate will automatically add to an array value with key 2 and value null).
Indexed values are in order, but select criteria filter part of them (e.g. your HQL similar to that "from user u join u.authorities a where a.id=2". In that case hibernate load a user, but in authorities array you will have only 3 values: 0 - null, 1 - null, 2 - authority with id 2).