I've been wrecking my mind on how to get my tagging of entities to
work. I'll get right into some database structuring:
tblTag
TagId - int32 - PK
Name
tblTagEntity
TagId - PK
EntityId - PK
EntityType - string - PK
tblImage
ImageId - int32 - PK
tblBlog
BlogId - int32 - PK
class Image
Id
EntityType { get { return "MyNamespace.Entities.Image"; }
IList<Tag> Tags;
class Blog
Id
EntityType { get { return "MyNamespace.Entities.Blog"; }
IList<Tag> Tags;
The obvious problem I have here is that EntityType is an identifer but
doesn't exist in the database. If anyone could help with the this
mapping I'd be very grateful.
You don't need the entity type. Take a look at any-type mapping (it stores the type name in the database in the relation table, but you don't need it in the entity model).
See this blog post by ayende.
Edit: tried to write an example.
You could have an own table for each tagged object, this is easy and straight forward, you don't even need any types:
<class name="Tag">
<!-- ... -->
<property name="Name"/>
</class>
<class name="Image">
<!-- ... -->
<bag name="Tags" table="Image_Tags">
<key column="Image_FK"/>
<many-to-many class="Tag" column="TagId "/>
</bag>
</class>
Tried to use some advanced features to map it into a single table, but I think it doesn't work this way:
<class name="Tag">
<!-- ... -->
<property name="Name"/>
<bag name="Objects" table="tblTagEntity" access="noop">
<key column="TagId"/>
<many-to-any id-type="System.Int64" meta-type="System.String">
<meta-value
value="IMAGE"
class="Image"/>
<meta-value
value="BLOG"
class="Blog"/>
<column name="EntityType"/>
<column name="EntityId"/>
</many-to-any>
</bag>
</class>
<class name="Image">
<!-- ... -->
<bag name="Tags" table="tblTagEntity" where="EntityType='IMAGE'">
<key column="EntityId"/>
<many-to-many class="Tag" column="TagId "/>
</bag>
</class>
The tricks here are:
access="noop" to specify the foreign key without having a property in the entity model, see this post.
where="EntityType='IMAGE'" to filter the loaded data.
The problem is that most probably the EntityType is not set to any useful value. This could be fixed somewhere, but I don't think that it is worth the effort.
Someone else has probably a better idea.
Edit 2: another (working) solution
make the association table an entity:
in short:
Tag => TagEntity: not mapped or one-to-many inverse (noop)
TagEntity => Tag: many-to-one
TagEntity => Object: any
Object => TagEntity: one-to-many inverse
This should work straight forward.
classes:
class Tag
{
string Name { get; set; }
}
class TagEntity
{
Tag Tag { get; set; }
object Entity { get; set; }
}
class Image
{
IList<TagEntity> tags { get; private set; }
}
The only drawback seems to be that you have to make sure that the bidirectional associations are consistent without loading to much data. Note that inverse collections are not stored.
Edit 2: Performance notes
When you add / remove tags, you could do a trick. TagEntity has a reference to the tagged entity. The Entity also has a list of TagEntities, but this is marked as inverse. (This means, they are loaded, but not stored.)
You can add and remove tags without loading the Entity an without loading all the tags.
Adding:
Get Tag to add (or load proxy if you have the id of the tag)
Load Entity (just proxy, using session.Load, no db access here)
create new TagEntity, assign tag and entity-proxy
save TagEntity
Removing:
Get TagEntity to remove
delete TagEntity.
Within the session, you don't have this tag assigned to/removed from the TagEntity. This works fine assumed that you only add or remove tags within this transaction.
I you define a list of TagEntities on the Tag, you can do the same, without loading all the TagEntities just to add or remove one.
You could make EntityType an Enum in your code. And/or, you could try making EntityType an actual entity in your database (tblEntityType).
Got Stefans final solution to work! Here's my final mappings:
Image
<bag name="TagEntites" table="tblTagEntity" cascade="all" fetch="join" inverse="true" where="EntityType='EntityImage'">
<key column="EntityId"></key>
<one-to-many class="TagEntity" />
</bag>
TagEntity
<id name="Id">
<column name="TagEntityId"></column>
<generator class="identity" />
</id>
<any name="Entity" id-type="System.Int32" meta-type="System.String">
<meta-value value="EntityImage" class="Image" />
<column name="EntityType"></column>
<column name="EntityId"></column>
</any>
<many-to-one name="Tag" class="Tag" cascade="all" fetch="join">
<column name="TagId"></column>
</many-to-one>
Related
How do I map relationship, where child endpoint is exposed via Id property and not via whole Parent object?
Here is the example:
class Parent {
public Guid Id { get; set; }
public List<Child> Chlidren { get; set; }
}
class Child {
public Guid Id { get; set; }
public Guid ParentId { get; set; }
}
Here are the equivalent mappings I'm using:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="Blabla"
namespace="Blabla"
auto-import="false">
<typedef name="ChildrenList" class="Blabla" />
<class name="Parent" table="Parent" lazy="false">
<id name="Id" column="ID" type="Guid">
<generator class="guid" />
</id>
<bag name="Children" table="Child"
cascade="save-update"
collection-type="ChildrenList"
lazy="false">
<key column="ParentID" not-null="true" />
<one-to-many class="Child" />
</bag>
</class>
<class name="Child" table="Child" lazy="false">
<id name="Id" column="ID" type="Guid">
<generator class="guid" />
</id>
<!-- How to map ParentID here? -->
</class>
</hibernate-mapping>
When I create a parent, add some children to Children collection and then save the parent, everything is fine. But if save a parent object first, then create a child, setting its ParentID property to ID of the parent, then I get
NHibernate.PropertyValueException:
not-null property references a null or transient value Child._Parent.ChildrenBackref
All attempts to map many-to-one relationship resulted in different exceptions while creating NHibernate configuration. Mostly about object type mismatch.
I'm sure NHibernate is capable to handle this scenario. There must something fairly basic that I miss.
EDIT:
I think it make sense to the example test, which fails with above exception:
var child = new Child(Create.Saved<Parent>().Id); // this sets the ParentId property
this.Repository.Save(child); // here I get the exception
My thoughts why NHibernate is raising this: Children property of Parent class mapped in a way that says that a child cannot exist without a parent (<key column="ParentID" not-null="true" />). When I try to persist a child, NHibernate tries to resolve this relationship (to find a parent this child relates to) and fails, since being given no child endpoint (which otherwise would be ParentId property) in the mapping, it check for its own Child._Parent.ChildrenBackref endpoint, whatever it is.
This looks like a desired solution: Mapping ParentId property as child endpoint of the relationship. This would force NHibernate to resolve a parent by using value of ParentId property as parent's primary key.
The thing is I don't know if it's possible.
The one-to-many / many-to-one relationships you have in NHibernate always needs to have a dominant side (i.e. the side that manages the "saving").
<bag name="Children" table="Child"
cascade="save-update"
collection-type="ChildrenList"
lazy="false">
<key column="ParentID" not-null="true" />
<one-to-many class="Child" />
</bag>
The above is a one-to-many relationship where the dominant side is the parent. That means, you save the parent ... and that will save the parent first, then, the children (with the ParentId being null), then a subsequent update will be issued to set the child.ParentId.
Note:
The child is inserted first with ParentId=null ... if you have a db or mapping restriction to say ParentId cannot be null, this action will fail.
<bag name="Children" table="Child"
cascade="save-update"
collection-type="ChildrenList"
lazy="false"
inverse=true>
<key column="ParentID" not-null="true" />
<one-to-many class="Child" />
</bag>
Note the inverse=true attribute. This means the child object is dominant in the relationship, meaning the child object is in charge. The parent will be inserted, then the Id will be assiged to the child.ParentId, and then the child will be inserted with the ParentId already set.
In many cases, of course, you want to go either way. The easiest way to do this is to manage the relationship on both ends (unfortunately, you have to do this yourself).
On the Parent, you have a method:
public void AddChild(Child child)
{
Children.Add(child);
child.ParentId = Id;
}
public void RemoveChild(Child child)
{
Children.Remove(child);
child.ParentId = null;
}
On the Child, you have a method:
public void SetParent(Parent parent)
{
ParentId = parent.Id;
parent.Children.Add(this);
}
Using these methods to Add/Remove/Set, both sides are consistent after the action is performed. It, then, wouldn't matter whether you set inverse=true on the bag or not.
see http://www.nhforge.org/doc/nh/en/index.html#collections-example
I have the following entities:
namespace NhLists {
public class Lesson {
public virtual int Id { get; set; }
public virtual string Title { get; set; }
}
public class Module {
public virtual int Id { get; set; }
public virtual IList<Lesson> Lessons { get; set; }
public Module() {
Lessons = new List<Lesson>();
}
}
}
And the following mappings:
<class name="Module" table="Modules">
<id name="Id">
<generator class="identity"/>
</id>
<list name="Lessons" table="ModuleToLesson"
cascade="save-update">
<key column="moduleId"/>
<index column="position"/>
<many-to-many
column="lessonId"
class="NhLists.Lesson, NhLists"/>
</list>
</class>
<class name="Lesson" table="Lessons">
<id name="Id">
<generator class="identity"/>
</id>
<property name="Title">
<column name="Title" length="16" not-null="true" />
</property>
</class>
When I delete a lesson by session.Delete(lesson), is there anyway I can have NHibernate automatically update the association in Module.Lessons to remove the entry from the set? Or am I forced to go through all Modules and look for the lesson and remove that by hand?
Edit: Fixed ICollection and <set> in mappings to IList<> and <list> like I want and tested it.
You have false idea. If you want to delete the Lesson object from Module you do that manually. NHibernate just tracks such your action and when session.Commit() is called then the reference between Module and Lesson is deleted in the database.
Calling session.Delete(lesson) deletes the lesson object from database (if foreign keys are set properly then reference between Module and Lesson is deleted of course but it is not responsibility for NHibernate).
In conclusion, it is not possible to delete the lesson object from the Module.Lessons list automatically by calling session.Delete(lesson). NHibernate does not track such entity references.
Turns out that if we do not need IList semantics and can make do with ICollection the update problem can be solved by adding a reference back from Lesson to Module, such as:
public class Lesson {
...
protected virtual ICollection<Module> InModules { get; set; }
...
}
And to the mapping files add:
<class name="Lesson" table="Lessons">
...
<set name="InModules" table="ModuleToLesson">
<key column="lessonId"/>
<many-to-many column="moduleId" class="NhLists.Module, NhLists"/>
</set>
</class>
Then a Lesson deleted is also removed from the collection in Module automatically. This also works for lists but the list index is not properly updated and causes "holes" in the list.
I'm working with an existing database that has the following structure. Changing the database schema is a last resort.
Products
Id
Name
ParentProducts
ParentId
ChildId
I don't want an entity for ParentProducts, I have the following for the children property (still need to test it, but that's the concept).
<bag name="Children" lazy="true" table="dbo.ParentProducts" cascade="save-update" inverse="true" >
<key column="[ChildId]"></key>
<many-to-many column="[ProductId]" class="Product" />
</bag>
What I'm struggling with is how do I create a Parent property? I'd like to do something like the following, but table isn't a valid attribute for many-to-one.
<many-to-one name="Parent" column="[ParentId]" table="dbo.ParentRelated" class="Policy" />
I could create a bag and only ever look at the first item, but that's more of a hack.
Any ideas?
Thanks
Creating a bag is the easiest solution. And you can give it a clean interface:
protected virtual ICollection<Product> Parents { get; set; }
public virtual Product Parent
{
get
{
return Parents.SingleOrDefault();
}
set
{
Parents.Clear();
Parents.Add(value);
}
}
With this, the rest of the code doesn't need to be aware of the DB/mapping structure.
Suppose I have a database like this:
This is set up to give role-wise menu permissions.
Please note that, User-table has no direct relationship with Permission-table.
Then how should I map this class against the database-tables?
class User
{
public int ID { get; set; }
public string Name { get; set; }
public string Username { get; set; }
public string Password { get; set; }
public bool? IsActive { get; set; }
public IList<Role> RoleItems { get; set; }
public IList<Permission> PermissionItems { get; set; }
public IList<string> MenuItemKeys { get; set; }
}
This means,
(1) Every user has some Roles.
(2) Every user has some Permissions (depending on to Roles).
(3) Every user has some permitted MenuItemKeys (according to Permissions).
How should my User.hbm.xml look like?
Roles and Permissions are likely to be accessed a lot in the application. They are very likely to be in the second level cache, which means we can expect to efficiently iterate the User.RoleItems and Role.Permissions.
This has the advantage that we can generally expect to perform no queries when iterating those collections.
You could map the classes as follows.
The properties User.PermissionItems and User.MenuItemKeys are derived from the persistent entities, and thus do not appear in the mappings.
<class name="User" table="user">
<id name="ID">
<generator class="native"/>
</id>
<property name="Name"/>
<property name="Username"/>
<property name="Password"/>
<property name="IsActive"/>
<bag name="RoleItems" table="userrole" lazy="true">
<key column="userid" />
<many-to-many class="Role" column="roleid"/>
</bag>
</class>
<class name="Role" table="role">
<id name="ID">
<generator class="native"/>
</id>
<property name="RoleName"/>
<property name="IsActive"/>
<bag name="Permissions" table="permission">
<key column="roleid" />
<one-to-many class="Permission"/>
</bag>
</class>
<class name="Permission" table="permission">
<id name="ID">
<generator class="native"/>
</id>
<property name="MenuItemKey"/>
</class>
I would make the 2 additional lists you had on User into derived enumerations. If they were lists, there is no unambiguous way to insert into them since you cannot know to which role the value applies. Also, a Role is not owned by a User.
Update: now using Diego's improved version of these properties.
class User
{
public virtual IEnumerable<Permission> PermissionItems
{
get {
return RoleItems.SelectMany(role => role.PermissionItems);
}
}
public virtual IEnumerable<string> MenuItemKeys
{
get {
return RoleItems.SelectMany(role => role.PermissionItems,
(role, permission) => permission.MenuItemKey);
}
}
}
The mapping posted by Lachlan is the best alternative. You could use queries that perform all the joins for each collection, but that'd make them read only for practical purposes.
There is a much easier way to implement the property code, however, that might help you decide:
public IEnumerable<Permission> PermissionItems
{
get
{
return RoleItems.SelectMany(role => role.PermissionItems);
}
}
public IEnumerable<string> MenuItemKeys
{
get
{
return RoleItems.SelectMany(role => role.PermissionItems,
(role, permission) => permission.MenuItemKey);
}
}
Here's a link: Chapter 6. Collection mapping
Here's another useful link: Chapter 7. Association Mappings
EDIT
After having reasearched for an entire evening, I came to the following conclusion:
Considering NHibernate Best Practices, what you wish to do is no good;
Don't use exotic association mappings.
Good usecases for a real many-to-many associations are rare. Most of the time you need additional information stored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate link class. In fact, we think that most associations are one-to-many and many-to-one, you should be careful when using any other association style and ask yourself if it is really neccessary.
As a programming philosophy, I prefer to keep it simple than having to write clever code where even me would no more understand what I wrote after a certain time;
Plus, I even considered using the subquery element of association mapping which would have worked if I would have found a way to parameterize it, if it is doable, but it seems it won't let me parameterize the query with the User instance's Id property value;
In the optic of a well designed OO model, a child being aware of his parent's properties is fine, but a parent accessing a child's property makes no sens - design smell;
As I may understand considering the context exposed the benefits of having permissions or MenuItemKey values accessible from the User directly, I suggest the following solution:
Create yourself a user defined dataview which will hold the values related to the MenuItemKey Permission attribute gotten through the Roles the User is a member like so:
CREATE VIEW udvUsersPermissions AS
SELECT UR.UserID, P.ID as N'ID', P.MenuItemKey
FROM Users U
INNER JOIN UsersRoles UR ON UR.UserID = U.ID
INNER JOIN Roles R ON R.ID = UR.RoleID
INNER JOIN Permissions P ON P.RoleID = R.ID
GO
Then, map it according in you User.hbm.xml file:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
<class name="User" table="Users">
<id name="Id" column="ID">
<generator class="identity"/>
</id>
<property name="Name" length="100"/>
<property name="UserName" length="10" not-null="true"/>
<property name="Password" length="10" not-null="true"/>
<property name="IsActive" not-null="true"/>
<list name="Roles" table="UsersRoles" access="private-property" lazy="true">
<key column="UserID" foreign-key="FK_UR_U"/>
<list-index column="UserID"/>
<many-to-many class="Role" column="RoleID" />
</list>
<!-- Here mapping Permissions granted to User. -->
<list name="Permissions" table="udvUsersPermissions" lazy="true">
<key column="UserID"/>
<list-index column="MenuItemKey"/>
<many-to-many column="ID" class="Permission"/>
</list>
</class>
</hibernate-mapping>
And here, I will let you know about the subselect solution, in case it works the way I didn't expect it to.
<list name="Permissions" lazy="true">
<subselect> <!-- see section 7.6, Chapter 7 - Association mappings -->
select U.ID, P.ID, P.MenuItemKey
from Users U
inner join UsersRoles UR ON UR.UserID = U.ID
inner join Roles R ON R.ID = UR.RoleID
inner join Permissions P ON P.RoleID = R.ID
group by U.ID, P.ID, P.MenuItemKey
order by P.MenuItemKey
</subselect>
<key column="U.ID"/>
<list-index column="P.MenuItemKey"/>
<many-to-many class="Permission" column="P.ID"/>
</list>
Now, I hope I brought enough details so that it helps you achieve what you want to do or either get on track. =)
My class has a field of type Dictionary<string, List<string>>. What's the best way to map it with NHibernate? I'd better leave it as a field, don't want to expose it.
Thanks a lot!
ulu
You can't directly map it. There are two rules to consider:
Always use interfaces for collections (eg. IList<T>, IDictionary<K,V>)
NH does not support nested collections. I've never seen an application for it before
and never heard someone requesting it.
Put your list of string into a class and use interfaces:
class StringList
{
IList<string> Strings { get; private set; }
}
class Entity
{
private IDictionary<string, StringList> stringDict;
}
You might even see some advantages of having such a class.
Mapping:
<class name="Entity">
...
<map name="stringDict" table="Entity_StringDict" access="field">
<key column="Entity_FK"/>
<index column="Key" type="System.String"/>
<composite-element class="StringList">
<bag name="Strings" table="Entity_StringDict_Strings">
<key column="Entity_StringDict_FK"/>
<element type="System.String" column="String"/>
</bag>
</composite-element>
</map>
</class>
Maps to three Tables:
Table Entity
Table Entity_StringDict
Column Entity_FK
Column Key
Table Entity_StringDict_Strings
Column Entity_StringDict_FK
Column String