Is a and has relationship

HAS-A relationship? (Beginning Java forum at Coderanch)

is a and has relationship

How to use a 'has a' relationship: Member Variable «Class «C# / CSharp Tutorial public Engine engine; // Car has an Engine public void Start() { engine. The has-many-through relationship provides a convenient short-cut for accessing distant relations via an intermediate. A-Kind-Of relationship; Is-A relationship; Part-Of relationship; Has-A relationship. Inheritance; Multiple Inheritance; Abstract Classes.

Now that we have examined the table structure for the relationship, let's define it on the Country model: Typical foreign key conventions will be used when performing the relationship's queries.

is a and has relationship

The key parameter is the name of the foreign key on the intermediate model, while the throughKey parameter is the name of the foreign key on the final model. One To One Table structure A one-to-one polymorphic relation is similar to a simple one-to-one relation; however, the target model can belong to more than one type of model on a single association.

For example, imagine you want to store photos for your staff members and for your products. Using polymorphic relationships, you can use a single photos table for both of these scenarios. First, let's examine the table structure required to build this relationship: Model structure Next, let's examine the model definitions needed to build this relationship: For example, to access the photo for a staff member, we can simply use the photo dynamic property: In our case, that is the imageable definition on the Photo model.

So, we will access it as a dynamic property: One To Many Table Structure A one-to-many polymorphic relation is similar to a simple one-to-many relation; however, the target model can belong to more than one type of model on a single association.

For example, imagine users of your application can "comment" on both posts and videos.

Similar Threads

Using polymorphic relationships, you may use a single comments table for both of these scenarios. For example, to access all of the comments for a post, we can use the comments dynamic property: In our case, that is the commentable definition on the Comment model.

Many To Many Table structure In addition to "one-to-one" and "one-to-many" relations, you may also define "many-to-many" polymorphic relations. For example, a blog Post and Video model could share a polymorphic relation to a Tag model.

is a and has relationship

Using a many-to-many polymorphic relation allows you to have a single list of unique tags that are shared across blog posts and videos. First, let's examine the table structure: So, for this example, we will define a posts relation and a videos relation: For example, to access all of the tags for a post, you can simply use the tags dynamic property: In our case, that is the posts or videos methods on the Tag model.

Relationship | Definition of Relationship by Merriam-Webster

So, you will access those relations as dynamic properties: Using a custom polymorphic type lets you decouple your database from your application's internal structure. You may define a relationship "morph map" to provide a custom name for each model instead of the class name: Querying relations Since all types of Model relationships can be called via functions, you may call those functions to obtain an instance of the relationship without actually executing the relationship queries.

In addition, all types of relationships also serve as query buildersallowing you to continue to chain constraints onto the relationship query before finally executing the SQL against your database.

For example, imagine a blog system in which a User model has many associated Post models: This gives you the ability to chain any of the query builder methods on the relationship.

  • View Unread Posts
  • Get the Latest Versions
  • IS-A (Inheritance) :

Because of this, developers often use eager loading to pre-load relationships they know will be accessed after loading the model. Eager loading provides a significant reduction in SQL queries that must be executed to load a model's relations.

Querying relationship existence When accessing the records for a model, you may wish to limit your results based on the existence of a relationship. For example, imagine you want to retrieve all blog posts that have at least one comment.

To do so, you may pass the name of the relationship to the has method: For example, you may retrieve all posts that have at least one comment and vote: These methods allow you to add customized constraints to a relationship constraint, such as checking the content of a comment: This means the relationship data is not actually loaded until you first access the property.

However, models can "eager load" relationships at the time you query the parent model. So, if we have 25 books, this loop would run 26 queries: Thankfully we can use eager loading to reduce this operation to just 2 queries. Create a relationship in an Access desktop database On the Database Tools tab, in the Relationships group, click Relationships.

If you haven't yet defined any relationships, the Show Table dialog box automatically appears.

Create a relationship

If it doesn't appear, on the Design tab, in the Relationships group, click Show Table. The Show Table dialog box displays all of the tables and queries in the database.

To see only tables, click Tables. Select one or more tables, and then click Add. After you have finished adding tables, click Close. Drag a field typically the primary key from one table to the common field the foreign key in the other table. To drag multiple fields, press the Ctrl key, click each field, and then drag them. The Edit Relationships dialog box appears.

Verify that the field names shown are the common fields for the relationship. If a field name is incorrect, click on the field name and select the appropriate field from the list. To enforce referential integrity for this relationship, select the Enforce Referential Integrity box. When you are finished in the Relationships window, click Save to save your relationship layout changes.

Access draws a relationship line between the two tables.

Java HAS-A relationship : aggregation - Java Tutorials - fabula-fantasia.info

If you selected the Enforce Referential Integrity check box, the line appears thicker at each end. This means that the Indexed property for these fields should be set to Yes No Duplicates.

If both fields have a unique index, Access creates a one-to-one relationship. This means that the Indexed property for this field should be set to Yes No Duplicates. The field on the many side should not have a unique index. It can have an index, but it must allow duplicates.

When one field has a unique index, and the other does not, Access creates a one-to-many relationship.