I have an one entity in my edmx model having an one property that can contains huge XML data.
Basically I want to load this entity without this property (column) /* huge data loading */ . And load this column only when it is strictly needed.
I have tried to create an inherited entity containing this property and remove this property from base entity (original entity). I have done mapping.
At this time I have problem, that during compilation a I get error, that base entity is not capable to insert and update itself, because property is not nullable
I am looking for best approach (solution) how this situation should be solved.
I am attaching the cut-out from my emdx designer (containing my current and desired situation)
UPDATE:
I will try to write a procedure that I have tried:
I mapped functions to my custom functions. For entity TRP_TechReport_T without the XML column (property). Then I just mapped for entity TRP_TechReport_T functions to my custom function (containing XML column).
Then I set Mapping condition on the entity TRP_TechReport_T: When TRP_XML = Empty.String
TechReport_T mappings:
TechReport_T functions:
TechReportFull_T mappings:
TechReportFull_T functions:
At this moment I get error:
Error 2 Error 3032: Problem in mapping fragments starting at line 3754:Condition member 'TRP_TechReport_T.TRP_XML' with a condition other than 'IsNull=False' is mapped. Either remove the condition on TRP_TechReport_T.TRP_XML or remove it from the mapping.
The column is not nullable in the database and mustn't be.
I can hard-set XML property to nullable, but in the case of the model updating from the database information will be lost.
At the moment it's the only thing I could think of.
Related
I have a table and a view which share the same columns and I'm trying to add them to EDM. I created an abstract entity type and 2 derived types A and B.
Initially I tried creating 2 entity sets, one for each of the derived types, and proceeded to add the EntitySetMapping (using IsTypeOf per MS MSL specifications for EntityTypeMapping of derived type) but got an error in the .edmx file that each of the types doesn't have an entity set and isn't mapped.
To solve this, I created a single EntitySet with type as the base EntityType (the abstract one) and used 2 EntityTypeMapping children tags, specifying the StoreEntitySet as the table or the view accordingly. This resolved the issue but created a single DbSet<BaseType> and now I cannot retrieve data from the view or the table using DbContext.
Any ideas on how to properly solve this?
Thanks
Using Entity Framework Core 2.0
Stuck with company's production database which has primary keys defined for each table but no foreign keys defined for any relationships.
Dependent records in the database have id fields which are intended to relate to the primary key fields of the parent record like you would normally find with a foreign key relationship/constraint. But these fields were all created as INT NOT NULL and are using a SQL default of '0'.
As a result dependent records have been inserted over time without requiring that a related parent record be specified.
Initially I defined my models in EF with integers and used a fluent configuration to specify "IsRequired". This was done so I could run migrations to create a test database for comparison against the production database to verify that my code first was correctly coded.
This then lead to the problem while using "Include" in my Linq queries which performs an inner join that results in dropping the records that contain the 0's in the id fields of the dependent record.
The only way that I have found to make this work is to model all of the id fields in the dependent entity as nullable integers and remove the "IsRequired" from the fluent configuration.
When using the "Include" it performs a left outer join keeping all of the dependent entities. This also means that any reference properties on the included entities are set to null instead of an empty string. This part can probably be fixed fairly easily.
The downside is if I wanted to use migrations to create a database now, all id fields in the dependent records would be created as NULL.
Is there anyone who has run up against this type of situation? Does anyone have any suggestions to try other than the approach I am using?
I haven't dealt with this scenario before but I wonder if you can solve it by defining the FK property as Nullable and then in the migrations, after the migration is created, edit it to add a HasDefaultValue property to ensure that it's 0? (doc for that migration method: https://learn.microsoft.com/en-us/ef/core/modeling/relational/default-values)
We have a customer very large table with over 500 columns (i know someone does that!)
Many of these columns are in fact foreign keys to other tables.
We also have the requirement to eager load some of the related tables.
Is there any way in Linq to SQL or Dynamic Linq to specify what columns to be retrieved from the database?
I am looking for a linq statement that actually HAS this effect on the generated SQL Statement:
SELECT Id, Name FROM Book
When we run the reguar query generated by EF, SQL Server throws an error that you have reached the maximum number of columns that can be selected within a query!!!
Any help is much appreciated!
Yes exactly this is the case, the table has 500 columns and is self referencing our tool automatically eager loads the first level relations and this hits the SQL limit on number of columns that can be queried.
I was hoping that I can set to only load limited columns of the related Entities such as Id and Name (which is used in the UI to view the record to user)
I guess the other option is to control what FK columns should be eager loaded. However this still remains problem for tables that has a binary or ntext column which you may not want to load all the times.
Is there a way to hook multiple models (Entities) to the same table in Code First? We tried doing this I think the effort failed miserably.
Yes you can return only subset of columns by using projection:
var result = from x in context.LargeTable
select new { x.Id, x.Name };
The problem: projection and eager loading doesn't work together. Once you start using projections or custom joins you are changing shape of the query and you cannot use Include (EF will ignore it). The only way in such scenario is to manually include relations in the projected result set:
var result = from x in context.LargeTable
select new {
Id = x.Id,
Name = x.Name,
// You can filter or project relations as well
RelatedEnitites = x.SomeRelation.Where(...)
};
You can also project to specific type BUT that specific type must not be mapped (so you cannot for example project to LargeTable entity from my sample). Projection to the mapped entity can be done only on materialized data in Linq-to-objects.
Edit:
There is probably some misunderstanding how EF works. EF works on top of entities - entity is what you have mapped. If you map 500 columns to the entity, EF simply use that entity as you defined it. It means that querying loads entity and persisting saves entity.
Why it works this way? Entity is considered as atomic data structure and its data can be loaded and tracked only once - that is a key feature for ability to correctly persist changes back to the database. It doesn't mean that you should not load only subset of columns if you need it but you should understand that loading subset of columns doesn't define your original entity - it is considered as arbitrary view on data in your entity. This view is not tracked and cannot be persisted back to database without some additional effort (simply because EF doesn't hold any information about the origin of the projection).
EF also place some additional constraints on the ability to map the entity
Each table can be normally mapped only once. Why? Again because mapping table multiple times to different entities can break ability to correctly persist those entities - for example if any non-key column is mapped twice and you load instance of both entities mapped to the same record, which of mapped values will you use during saving changes?
There are two exceptions which allow you mapping table multiple times
Table per hierarchy inheritance - this is a mapping where table can contains records from multiple entity types defined in inheritance hierarchy. Columns mapped to the base entity in the hierarchy must be shared by all entities. Every derived entity type can have its own columns mapped to its specific properties (other entity types have these columns always empty). It is not possible to share column for derived properties among multiple entities. There must also be one additional column called discriminator telling EF which entity type is stored in the record - this columns cannot be mapped as property because it is already mapped as type discriminator.
Table splitting - this is direct solution for the single table mapping limitation. It allows you to split table into multiple entities with some constraints:
There must be one-to-one relation between entities. You have one central entity used to load the core data and all other entities are accessible through navigation properties from this entity. Eager loading, lazy loading and explicit loading works normally.
The relation is real 1-1 so both parts or relation must always exists.
Entities must not share any property except the key - this constraint will solve the initial problem because each modifiable property is mapped only once
Every entity from the split table must have a mapped key property
Insertion requires whole object graph to be populated because other entities can contain mapped required columns
Linq-to-Sql also contains ability to mark a column as lazy loaded but this feature is currently not available in EF - you can vote for that feature.
It leads to your options for optimization
Use projections to get read-only "view" for entity
You can do that in Linq query as I showed in the previous part of this answer
You can create database view and map it as a new "entity"
In EDMX you can also use Defining query or Query view to encapsulate either SQL or ESQL projection in your mapping
Use table splitting
EDMX allows you splitting table to many entities without any problem
Code first allows you splitting table as well but there are some problems when you split table to more than two entities (I think it requires each entity type to have navigation property to all other entity types from split table - that makes it really hard to use).
Create stored procedures that query the number of columns needed and then call the stored procs from code.
I have very big table in my Database and a can't modify it.
So i have BaseEntity type for table.
I have several children (entity1, entity2) and i'd like to map each type to same column ("Date") and name properties differently.
Surely i can't move all same column properties to base type cause there is about 100 columns in my super table (it's not my design i've jst need to map it)
So i have 0019 error and is there any way to solve it or EF not for me?
No. TPH requires that each property defined in derived entity is exclusive for that entity (no other entity can map to the same column). This targets more general rule in EF - each column can be mapped only once. So if you need to use some column in more entities it must be defined in parent and must have same name in all child entities.
I am trying to create a custom Entity Framework (4.2) entity that would be mapped to my database like it would be done in a Code first approach.
The issue is that my entity framework data model is using Database first.
How can I add my custom entity to entity framework's context?
If by the Database first you mean that you already have EDMX created from exiting database you simply cannot use code first. You must create table and update model (EDMX) from the database to include it in EDMX.
Edit based on comment:
I want to create a BriefUser entity that would basically be a lighter
version of User but it would have properties retrieved from User's
foreign keys.
Well this is possible. You can either create BriefUser as common class and use projection in query.
var breifUser = (from x in context.Users
where ...
select new BriefUser
{
// Fill BreifUser's properties here
}).FirstOrDefault();
You can even refactor former code to reusable extension method:
public static IQueryable<BriefUser> ProjectUser(this IQueryable<User> query)
{
return query.Select(x => new BreifUser()
{ // Fill BreifUser's properties here });
}
and use it like:
var briefUser = context.Users.ProjectUser().FirstOrDefault(...);
It is also possible to define your new class as "entity view". The first problem is that each table can be mapped to only one entity (except some advanced concepts like inheritance or splitting) so you cannot define your BriefUser as a new entity type because mapping both User and BriefUser to UserTbl would violate this rule. You must use special construct called QueryView.
QueryView is view in mapping level. It allows you to create new mapped type which is projection of existing mapped entities defined directly in EDMX's MSL part. The projection is defined as custom Entity SQL query. The problem is that QueryView has limitations:
It doesn't offer all Entity SQL features - for example it doesn't support aggregations (which I consider as really missing feature). Without aggregations you for example cannot create a new type which will contain property counting some related entities.
It is not supported in designer. You must edit your EDMX as XML to define QueryView and you must write Entity SQL query yourselves.
Resulting type is a "view" and it is read-only.
I want to keep the EDMX file, but also be able to add an entity
(BriefUser) to EF's context.
This is not possible. Your BreifUser is only projection / view and EF is not able to track changes back to originating tables so you cannot add BreifUser to context and persist it. In case of QueryView you can achieve it if you define custom stored procedures which will no how to decompose BreifUser and modify all related tables. These stored procedures must be imported to the EDMX and mapped to data modification operations of the view entity. Btw. same will happen if you map your entity to the database view because EF takes all views as read-only.