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- Dictionaryunique/juːˈniːk/
adjective
- 1. being the only one of its kind; unlike anything else: "this discovery was unique in history" Similar Opposite
noun
- 1. a unique person or thing: archaic "some of Lamb's writings were so memorably beautiful as to be uniques in their class"
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Using [Index (nameof (<key column>), IsUnique = true)] it is possible to define indices on the entity class. Also if you want to create Unique constrains on multiple columns you can simply do this (following @Sampath's link) Since Entity Framework Core (EF Core) 5.0 we can configure Unique Indexes via Data Annotations.
Sep 19, 2013 · The second approach to create Unique Constraints with EF Core by using Alternate Keys. Examples. One column: Multiple columns: EF 6 and below: First approach: @"CREATE UNIQUE INDEX LX_{0} ON {0} ({1})", "Entitys", "FirstColumn, SecondColumn")); This approach is very fast and useful but the main problem is that Entity Framework doesn't know ...
To add a unique constraint, you need to use two components: ALTER TABLE - to change the table schema and, ADD UNIQUE - to add the unique constraint. You then can define your new unique key with the format 'name' ('column1', 'column2'...) So for your particular issue, you could use this command:
Mar 28, 2018 · In EF Core you could use the extension method HasAlternateKey in fluent API only. There are no data annotations to realize a unique constraint. This MS doc article - Alternate Keys (Unique Constraints) - will explain how to use and which further possibilities are exist. A short example from link above: class MyContext : DbContext.
Jun 3, 2013 · unique_ptr<double> uptr2 (pd); // ... unique_ptr<double> uptr3 (pd); Thus having two unique pointers that effectively encapsulate the same object (thus violating the semantics of a unique pointer). This is why the first form for creating a unique pointer is better, when possible. Notice, that in C++14 we will be able to do:
Downside to this solution is that it doubles the overhead of every unique_ptr (they must all store the function pointer along with the pointer to the actual data), requires passing the destruction function every time, it can't inline (since the template can't specialize to the specific function, only the signature), and must call the function through the pointer (more costly than direct call).
204. As said before, @Column(unique = true) is a shortcut to UniqueConstraint when it is only a single field. From the example you gave, there is a huge difference between both. @Column(unique = true) @ManyToOne(optional = false, fetch = FetchType.EAGER) private ProductSerialMask mask; @Column(unique = true)
Using the constraints features UniqueConstraint is preferred over unique_together. From the Django documentation for unique_together: Use UniqueConstraint with the constraints option instead. UniqueConstraint provides more functionality than unique_together. unique_together may be deprecated in the future. For example:
Apr 25, 2015 · Primary Key. 1)can't insert null value. 2) one table have one primary key. Unique key 1) insert null value one at time 2)one table have multiple unique key 3) you can refereed as candidate key also. foreign key 1) maintain the relationship between two table and also multiple Note: without any constraint you get data in multiple table but you ...
Unless your app is single-user, I believe a unique constraint is one thing you can't enforce with code alone. . You can dramatically reduce the probability of a violation in code (by checking uniqueness prior to calling SaveChanges()), but there's still the possibility of another insert/update slipping in between the time of the uniqueness check and the time of SaveChange
