I am having a probelm with web2py foreign keys, as this is lack of documentation on this its pretty fustrating. I define tables in different files most tables are related. I used to use db.table_name to denote foreign keys, but I was told to use 'reference tbl_name'. This however makes no difference whatsoever, I still get errors compalaining about models defined in later files as per alphabetical order rules for web2py. It seems it actually matters having tables in order, rendering the use of reference key word useless at best. Or am I missing somethng here.
I would assume you are using mysql. In such case, it is a mysql error. mysql raises error no 150 (foreign key error). So, it is a database backend issue rather than a web2py problem. Please do check the documentation for your database server. How I solved the issue is in the db.py file declare (db.define_table) all the primary tables that require no foreign keys first and then continue declaring the tables that require the foreign keys (Just ensure that the foreign table exists before referencing it). In case you are using individual files for different tables in the models folder, then try finding out which tables are primary and make sure that they are created first by prepending the filenames with numbers so that before referencing a foreign key in a table, the 'foreign' table will already be existing in the database.
However, there should be no issues with sqlite.
Related
I'm learning SQLAlchemy and using a database where multiple table lookups are needed to find a single piece of data.
I'm trying to find the best (most efficient and Pythonic) way to map the multiple lookups to a single SQLAlchemy object or reusable python method.
Ultimately, there will be dozens if not hundreds of mapped object such as these, so something like a .map file might be handy.
I.e. (Using pseudocode)
If I want to find the data 'Status' from 'Patient Name' have to use three tables.
Instead of writing a function for every potential 'this' to 'that' data request, is there an SQLAlchemy or Pythonic way to make the mappings?
I CAN make new, temporary SQLAlchemy Tables to store data. I am NOT at liberty to change the database I'm reading from. I'm hoping to reduce the number of individual calls to the database, because it is remote and slow.
I'm not sure a data join will work, because the Primary Keys, Foreign Keys and Column names are inconsistent in the database. But, I don't really know how to make select-joins in SQLAlchemy.
Perhaps I need to create a new table, with relationships to those three previous tables? But I'm not understanding the relationships well.
Can these tables be auto-generated from a map.ini file?
EDIT:
I might add, that some of these relationships could be one to many. I.e. a patient may be associated with more than one statusID...such as...
It says here that foreignkeys across databases can't be done because of referential integrity: https://docs.djangoproject.com/en/1.9/topics/db/multi-db/#cross-database-relations
Could this be overcome by using UUID's for the primary key?
I'm guessing either I don't fully understand referential integrity, or I'm not the first to think of this and it can't be done for reasons I don't know yet.
A foreign key means "make sure a value in this column exists in another column". This doesn't work cross-database in PostgreSQL, as databases cannot "see" each other. It does work cross-schema though - use the fully qualified schema_name.table_name.
Why are you trying to refer to a column in another database?
Simply put, i want to create a structure that has a component MAKTX, and to have a foreign key relation with MAKT-MAKTX.
More generally i want to have a foreign key check for a field that's not part of a primary key.
I see the button "Non-key-fields/candidates", but i don't really know how to use it.
Also, i don't want to use the "key fields of a text table" relation... but i don't know if that's relevant.
Is this even a good thing that i'm trying to do? I don't see any reason why it shouldn't be possible, but you might object.
[EDIT]: I have to mention that I don't really know what I'm doing. I really just want to fill a table i created with values from another, and to make sure that those values (namely MAKTX - kind ofvalues) in my table are always values from MAKT. Suppose i do the initial filling with a SELECT statement, i want the consistency to work even if i later insert new entries manually.
So I don't know whether this makes sense or not, it just sounds to me like a good idea to have the system perform this check automatically, if possible.
Main condition for creating foreign key relation is that the field should be a primary key in your reference table. While in the table you are creating foreign key its not necessary that the field is a primary key or not. The main reason for this is that foreign key cant be null.
Refer to below link for step by step process for creating foreign key relation in abap.
http://learnabaponline.blogspot.in/2013/04/how-to-create-table-in-abap.html
First off, I agree with vwegerts's comments, what you're trying to do doesn't seem to make any sense.
Perhaps this would make more sense: create your own table without the MAKTX field. Then create a database view, joining your table and the MAKT table (and set a default language in the selection conditions if you want to). This way you'll have the descriptions joined with your data, without duplicating the actual data (which is what it looks like you're trying to do).
I wanted to create an email authenticated django user model, and I basically followed the steps in this website:
http://www.micahcarrick.com/django-email-authentication.html
And also included the table alteration code in a post_syncdb function in a managmenet module, to make the email a unique identifier. This should work ok with MySql. BUT, it wont work for sqlite. This is because sqlite's table alteration is limited and wont allow you to change that attribute OR even add a column with a unique identifier.
If there is no elegant way of doing this, then I might have to switch to MySql.
http://www.sqlite.org/faq.html#q26
So, it UNIQUE is fully supported, but you cannot alter a table using UNIQUE. So dump the table to a new table that has the UNIQUE constraint then alter and rename the tables. Or just dump it, modify the dump and reimport it.
I think, in your post_syncdb hook, you can add:
cursor.execute(
"CREATE UNIQUE INDEX IF NOT EXISTS auth_user_email_unique "
"ON auth_user (email COLLATE NOCASE);"
)
you may have to break out different blocks based on settings.DATABASES['default']['ENGINE']
I'm reading that I can use raw SQL in Django and have Django actually build my models from the results.
However I'm wondering what happens if I use joins in the raw SQL. How will Django know what models to use?
(Are there any other issues I should be aware of?)
It's not the joins that matter, but the column names. You could, for example, do the following:
SELECT table.id, other_table.name AS name from table join other_table using (id)
and pass that into your table model. Django would then treat the names from other_table as though they were names from table and give your normal table instances. I can't imagine why you would want to do that though...
The important thing to remember is that Django is using a very simple mapping from your SQL to its model structure. You can subvert it if you want, but you'll probably end up with some hard to maintain code.