i have two tables
asset employee
assetid-pk empid-pk
empid-fk
now, i have a form to populate the asset table but it cant because of the foreign key constraint..
what to do?
thx
Tk
Foreign keys are created for a good reason - to prevent orphan rows at a minimum. Create the corresponding parent and then use the appropriate value as the foreign key value on the child table.
You should think about this update as a series of SQL statements, not just one statement. You'll process the statements in order of dependency, see example.
Asset
PK AssetID
AssetName
FK EmployeeID
etc...
Employee
PK EmployeeID
EmployeeName
etc...
If you want to "add" a new asset, you'll first need to know which employee it will be assigned to. If it will be assigned to a new employee, you'll need to add them first.
Here is an example of adding a asset named 'BOOK' for a new employee named 'Zach'.
DECLARE #EmployeeFK AS INT;
INSERT (EmployeeName) VALUES ('Zach') INTO EMPLOYEE;
SELECT #EmployeeFK = ##IDENTITY;
INSERT (AssetName, EmployeeID) VALUES ('BOOK',#EmployeeFK) INTO ASSET;
The important thing to notice above, is that we grab the new identity (aka: EmployeeID) assigned to 'Zach', so we can use it when we add the new asset.
If I understand you correctly, are you trying to build the data graph locally before persisting to the data? That is, create the parent and child records within the application and persist it all at once?
There are a couple approaches to this. One approach people take is to use GUIDs as the unique identifiers for the data. That way you don't need to get the next ID from the database, you can just create the graph locally and persist the whole thing. There's been a debate on this approach between software and database for a long time, because while it makes a lot of sense in many ways (hit the database less often, maintain relationships before persisting, uniquely identify data across systems) it turns out to be a significant resource hit on the database.
Another approach is to use an ORM that will handle the persistence mapping for you. Something like NHibernate, for example. You would create your parent object and the child objects would just be properties on that. They wouldn't have any concept of foreign keys and IDs and such, they'd just be objects in code related by being set as properties on each other (such as a "blog post" object with a generic collection of "comment" objects, etc.). This graph would be handed off to the ORM which would use its knowledge of the mapping between the objects and the persistence to send it off to the database in the correct order, perhaps giving back the same object but with ID numbers populated.
Or is this not what you're asking? It's a little unclear, to be honest.
Related
I'm using Sails.js to build an API for an existing database. Unfortunately, modifying the structure of the database is not an option.
Many tables in the database have status columns of one type or another. They tend to have single-letter values that don't make sense without context. Context is provided by a "lookup" table in the database with 3 primary keys: table_name, column_name, and column_contents. Therefore, if I have a letter returned as a status, I can do a query against the lookup table and check a fourth column, description.
I'd love to configure my Sails.js models to understand all this, but it seems that one-to-many relationships can only be set up for tables with a single primary key. Is that correct?
Based on the "many-to-many" workaround, I assume the sails way to solve this would be to create new tables that are subsets of the "lookup" table (each for a single instance of table_name, column_name). Is there a better way?
What is the Amazon-recommended way of changing the schema of a large table in a production DynamoDB?
Imagine a hypothetical case where we have a table Person, with primary hash key SSN. This table may contain 10 million items.
Now the news comes that due to the critical volume of identity thefts, the government of this hypothetical country has introduced another personal identification: Unique Personal Identifier, or UPI.
We have to add an UPI column and change the schema of the Person table, so that now the primary hash key is UPI. We want to support for some time both the current system, which uses SSN and the new system, which uses UPI, thus we need both these two columns to co-exist in the Person table.
What is the Amazon-recommended way to do this schema change?
There are a couple of approaches, but first you must understand that you cannot change the schema of an existing table. To get a different schema, you have to create a new table. You may be able to reuse your existing table, but the result would be the same as if you created a different table.
Lazy migration to the same table, without Streams. Every time you modify an entry in the Person table, create a new item in the Person table using UPI and not SSN as the value for the hash key, and delete the old item keyed at SSN. This assumes that UPI draws from a different range of values than SSN. If SSN looks like XXX-XX-XXXX, then as long as UPI has a different number of digits than SSN, then you will never have an overlap.
Lazy migration to the same table, using Streams. When streams becomes generally available, you will be able to turn on a Stream for your Person table. Create a stream with the NEW_AND_OLD_IMAGES stream view type, and whenever you detect a change to an item that adds a UPI to an existing person in the Person table, create a Lambda function that removes the person keyed at SSN and add a person with the same attributes keyed at UPI. This approach has race conditions that can be mitigated by adding an atomic counter-version attribute to the item and conditioning the DeleteItem call on the version attribute.
Preemptive (scripted) migration to a different table, using Streams. Run a script that scans your table and adds a unique UPI to each Person-item in the Person table. Create a stream on Person table with the NEW_AND_OLD_IMAGES stream view type and subscribe a lambda function to that stream that writes all the new Persons in a new Person_UPI table when the lambda function detects that a Person with a UPI was changed or when a Person had a UPI added. Mutations on the base table usually take hundreds of milliseconds to appear in a stream as stream records, so you can do a hot failover to the new Person_UPI table in your application. Reject requests for a few seconds, point your application to the Person_UPI table during that time, and re-enable requests.
DynamoDB streams enable us to migrate tables without any downtime. I've done this to great effective, and the steps I've followed are:
Create a new table (let us call this NewTable), with the desired key structure, LSIs, GSIs.
Enable DynamoDB Streams on the original table
Associate a Lambda to the Stream, which pushes the record into NewTable. (This Lambda should trim off the migration flag in Step 5)
[Optional] Create a GSI on the original table to speed up scanning items. Ensure this GSI only has attributes: Primary Key, and Migrated (See Step 5).
Scan the GSI created in the previous step (or entire table) and use the following Filter:
FilterExpression = "attribute_not_exists(Migrated)"
Update each item in the table with a migrate flag (ie: “Migrated”: { “S”: “0” }, which sends it to the DynamoDB Streams (using UpdateItem API, to ensure no data loss occurs).
NOTE: You may want to increase write capacity units on the table during the updates.
The Lambda will pick up all items, trim off the Migrated flag and push it into NewTable.
Once all items have been migrated, repoint the code to the new table
Remove original table, and Lambda function once happy all is good.
Following these steps should ensure you have no data loss and no downtime.
I've documented this on my blog, with code to assist:
https://www.abhayachauhan.com/2018/01/dynamodb-changing-table-schema/
I'm using a variant of Alexander's third approach. Again, you create a new table that will be updated as the old table is updated. The difference is that you use code in the existing service to write to both tables while you're transitioning instead of using a lambda function. You may have custom persistence code that you don't want to reproduce in a temporary lambda function and it's likely that you'll have to write the service code for this new table anyway. Depending on your architecture, you may even be able to switch to the new table without downtime.
However, the nice part about using a lambda function is that any load introduced by additional writes to the new table would be on the lambda, not the service.
If the changes involve changing the partition key, you can add a new GSI (global secondary index). Moreover, you can always add new columns/attributes to DynamoDB without needing to migrate tables.
I'm having a hard time debugging a particular problem and have a couple questions. First, here is what's going on:
I have a relatively simple table called Employees, which has a primary key / identity Id. There is also a Username column - which is a GUID foreign key to my aspnet_Users table used for membership. Finally, there is another foreign key Team_Id which points to another table, Teams.
All I'm really trying to do is give a selected employee's Id and pass it to a method in the DAL which then finds the employee with the following statement:
var employee = entities.Employees.Where(emp => emp.Id == employeeId);
Once the employee is retrieved, I want to use another value which is passed to the same method - the selected team's Id - to update the employee's Team_Id value (which team they are assigned to), using the following:
employee.First().Team_Id = teamId;
entities.SaveChanges();
I get the exception
Invalid column name: {Name}
which doesn't make sense to me, because Employee doesn't have a name column.
All of that said, my questions are:
Where could the mix up possibly be coming from? I've tried thinking up a way to step through the code, but it seems like the error is somewhere in the query itself so I'm not really sure how to trace the execution of the query itself.
Is it possible that it may have something to do with my generated Entities? I noticed that when I type employee.First(). Name comes up in Intellisense. I'm really confused by that, since as I've mentioned there is no Name column in the employees table.
Fixed the issue. I just removed the existing Entity Framework Model and re-added it.
As far as the query goes, you can always use SQL Profiler to watch what scripts are actually running. That's a good way to troubleshoot generated SQL anyway.
For your property, somehow that did make it to your class, so your data model thinks it's there, for whatever reason. I'd say just go to your data model (you don't mention if this this is EF or LINQ-to-SQL), and you'll see "Name" there. Just remove it, and it will remove it from the class, and from the data access stuff.
I am using a global application user account to access database A. This user account does not have permissions to modify database A's schema (ie, create tables, modify tables, etc). This user also has access to database B, but only views. I need to run SQL to feed data from a view in database B into a table in database A.
In a perfect world, I would be able to use this SQL:
create database_a.mytable as (select * from database_b) with no data
However, the user can't create tables in database A. If I could get the DDL of the select statement then I could log in under my personal account (which doesn't have any access to database B) and run the DDL in database A to create the table.
The only other option is to manually write the SQL, but I don't want to do that, especially since this view I am wanting to copy has many columns of varying data types and sizes.
Edit: I may be getting closer. I just experimented with this:
show (select * from database_b.myview)
However, it generated the DLL of every single table that is used in the view itself, as well as the definition for the view. This doesn't really help me since I just want the schema of the select statement itself. In other words, I need what would be generated if I were to use the create table as statement mentioned above.
Edit for Rob: Perhaps "DDL" was the wrong term to use. Using show view db.myview just shows the definition of the view, not the schema it represents. In my above example of create table as, I show how you can create a table that mimics the schema of a result set returned in a select. It generates a DDL on the back end for creating a table and then executes that DDL to actually create the table. You can then say show table db.newtable and see the new table's DDL. I want to get that DDL directly from a select statement so that I can copy it, log out of the app account, into my personal account, and then execute the DDL to create the table.
This is only to save me the headache of having to type out the DDL manually by hand to save time and reduce typing errors, especially since the source view has so many columns. That said, I think hitting up the DBA or writing some snazzy stored procedure to do dynamic stuff would be a bit over the top for my needs. I think there has to be a way to get the DDL for creating a table schema directly from a select statement.
Generate DDL Statements for objects:
SHOW TABLE {DatabaseB}.{Table1};
SHOW VIEW {DatabaseB}.{View1};
Breakdown of columns in a view:
HELP VIEW {DatabaseB}.{View1};
However, without the ability to create the object in the target database DatabaseA your don't have much leverage. Obviously, if the object already existed INSERT INTO SELECT ... FROM DatabaseB.Table1 or MERGE INTO would be options that you already explored.
Alternative Solution
Would it be possible to have a stored procedure created that dynamically created the table based on the view name that is provided? The global application account would simply need privilege to execute the procedure. Generally the user creating the stored procedure would need the permissions to perform the actions contained within the stored procedure. (You have some additional flexibility with this in Teradata 13.10.)
There are some caveats with this approach. You are attempting to materialize views that could reference anywhere from hundreds to billions of records. These aren't simple 1:1 views that are put on top of the target tables. Trying to determine the required space in the target database to materialize the view will be difficult. Performance can and will vary depending on the complexity of the view and the data volumes. This will not be a fast-path or data block optimized operation.
As a DBA, I would be concerned with this approach being taken on by a global application account without fully understanding the intent. I trust you have an open line of communication with the DBA(s) involved for supporting this system. I'm sure there are reasons for your madness that can't be disclosed here.
Possible Solution - VOLATILE TABLE
Unless the implicit privilege for CREATE TABLE has been revoked from the global application account this solution should work.
Volatile tables do not require perm space. There table definitions persist for the duration of the session and any data inserted into them relies on the spool space of the user who instantiated it.
CREATE VOLATILE TABLE {Global Application UserID}.{TableA_Copy} AS
(
SELECT *
FROM {DatabaseB}.{TableA}
)
WITH NO DATA
NO PRIMARY INDEX
ON COMMIT PRESERVE ROWS;
SHOW TABLE {Global Application UserID}.{TableA_Copy};
I opted to use a Teradata 13.10 feature called NO PRIMARY INDEX. By default, CREATE TABLE AS will take the first column of the SELECT statement and make it the PRIMARY INDEX of the table. This could lead to skewing and perm space issues in your testing depending on the data demographics. You can specify an explicit PRIMARY INDEX on your own as you understand the underlying data. (See the DDL manuals for details on the syntax if you're uncertain.)
The use of ON COMMIT PRESERVE ROWS for the intent of this example is probably extraneous. But in reality if you popped any data into that table for testing this clause would be beneficial in Teradata mode as the data would otherwise be lost immediately after the CREATE TABLE or any other data manipulation was performed against the volatile table.
I have web application where Iam using linq to business entites i have business data model.
the problem is :
I have table with one column that it dosen't allow null value, when I try to update this table the folloeing error arise:
error The property 'e.g Carrier' is part of the object's key information and cannot be modified
what I can do?
The easiest thing to do is add a second column to the table that has a unique key eg guid and create a read only property on the entity that corresponds to it.
Linq to business entites needs some kind of key to keep track of what to update in the database. Usually this is the primary key on the database table. If you dont have a primary key it cannot reliably update the database and will then send you an exception.
Also if there is no primary key explicitly set on the table linq to business entites will select one of the columns (think its the first column in the table but i could be wrong) to act as a primary key and will therefore not allow you to update it.