linq to SQLite

发布时间：2020-12-12 19:42:53 所属栏目：百科来源：网络整理

导读：https://www.devart.com/dotconnect/sqlite/articles/tutorial_linq.html LINQ to SQLite Tutorial This tutorial guides you through the process of creating a simple application powered byLinqConnecttechnology. In less than 5 minutes you will hav

CrmDemoDataContext context = new CrmDemoDataContext();var query = from it in context.Companiesorderby it.CompanyIDselect it;foreach (Company comp query)Console.WriteLine("{0} | {1} | {2}",comp.CompanyID,comp.CompanyName,comp.Country);Console.ReadLine();Visual Basic Dim context As CrmDemoDataContext = New CrmDemoDataContext query = From it In context.companies _ Order By it.CompanyID _ Select it comp company For Each query Console.WriteLine( "{0} | {1} | {2}" Next Console.ReadLine()

As simple as that. You prepare a query and then iterate through it as you would do with a usual collection of objects. The database interaction is performed by LinqConnect in the background. Now let's see who is who in this code sample.

CrmDemoDataContextis the name of the class that knows all about your model and does everything to retrieve and modify related data in the database. All LinqConnect operations are performed within this class's properties and methods. This class is designed to be lightweight and not expensive to create,thus it is recommended to create a new DataContext instance for any 'unit of work' and dispose it after this unit is completed.
query,itare arbitrary variable names in the LINQ to SQL statement. The former is used as the collection of data objects,the latter is used to reference single entities in a collection and exists inside the statement only.
context.Companiesrefers to a public property of CrmDemoDataContext class. This property represents the collection of all companies in the context.
Company(in theforeachstatement) is the name of an autogenerated class. This class maps to theCompanytable in the database and is named after it.

Here is the project's output in the console:

Note that the LINQ query code just describes the query. It does not execute it. This approach is known as deferred execution.

Now let's query data from two tables united with a foreign key. Replace the old code with this:

query) { if (comp.PersonContacts.Count > 0) { comp.CompanyName,comp.PersonContacts[0].FirstName, comp.PersonContacts[0].LastName); } }

If

comp.personcontacts.Count > 0 Then comp.personcontacts(0).LastName) End If Next

 As you can see,the LINQ query statement was not changed at all. The data about the contact persons was retrieved from the database automatically when you accessed the corresponding property of the company object. This is one of the great things about LINQ: you do not have to worry about dependencies when writing queries. 
 Inserting New Data 
 What earlier was adding rows to tables,now is just adding new objects to context collections. When you are ready to send the changes to the database,call SubmitChanges() method of the context. Before doing this,you must first set all properties that do not supportnull(Nothing) values. The SubmitChanges() method generates and executes commands that perform the equivalent INSERT,UPDATE,or DELETE statements against the data source. 
 Let's add a new product and a new category to the database. Replace the old code with this: 
 
  CrmDemoDataContext();

// Create a new category ProductCategory newCategory = ProductCategory(); newCategory.CategoryID = 1000; newCategory.CategoryName = "New category" ; // Create a new product Product newProduct = Product(); newProduct.ProductID = 2000; newProduct.ProductName = "New product" ; newProduct.Price = 20; // Associate the new product with the new category newProduct.ProductCategory = newCategory; context.Products.InsertOnSubmit(newProduct); // Send the changes to the database. // Until you do it,the changes are cached on the client side. context.SubmitChanges(); // Request the new product from the database context.Products where it.ProductID == 2000 select it; // Since we query for a single object instead of a collection,we can use the method First() Product product = query.First(); product.ProductCategory.CategoryName,product.ProductName,product.Price);

 
  CrmDemoDataContext

' Create a new category newCategory productcategory = productcategory() newCategory.CategoryID = 1000 newCategory.CategoryName = "New category" ' Create a new product newProduct product = product() newProduct.ProductID = 2000 newProduct.ProductName = "New product" newProduct.Price = 20 ' Associate the new product with the new category newProduct.productcategory = newCategory context.products.InsertOnSubmit(newProduct) ' Send the changes to the database. ' Until you do it,the changes are cached on the client side. context.SubmitChanges() ' Request the new product from the database context.products _ Where it.ProductID = 2000 _ it ' Since we query for a single object instead of a collection,we can use the method First() product product = query.First() product.productcategory.CategoryName,product.Price)

 The InsertOnSubmit() method is created for every collection in the context. This method stores in the database information about all linked objects. As shown in the example,it is only necessary to call InsertOnSubmit() once to submit both product and category objects. 
 Note that after you have added the new product and category by submitting the changes,you cannot execute this solution again as is. To execute the solution again,change the IDs of the objects to be added. 
 Updating Data 
 Entity instances are modified as usual. The only thing to remember is that you have to invoke the SubmitChanges() method to send the data to the database. 
 Append the following block to the existing code and launch the project: 
 
 product.ProductName =

"Edited product" ; product.Price = 15; context.SubmitChanges(); product.ProductName = "Edited product" product.Price = 15 context.SubmitChanges()

Deleting Data

To extract an instance from a context use the DeleteOnSubmit method of the corresponding collection. The object is removed from the collection of its type,but not destroyed. To delete the object's data from the database invoke the SubmitChanges() method.

You can do this with a block of code like the following:

context.products.DeleteOnSubmit(newProduct); context.productcategories.DeleteOnSubmit(newCategory);

 
 context.products.DeleteOnSubmit(newProduct)

context.productcategories.DeleteOnSubmit(newCategory) context.SubmitChanges()

Deletion of objects is affected by attributes in the model. WhenDeleteRuleparameter isCascade,dependent objects are deleted automatically. When this parameter isSetNull,dependent objects are not deleted,but the relation is nullified. When no rule is specified,the order of deletion sequence is important.

Additional Information

Now that you can perform the basic data manipulation withLinqConnect,you can move on to some advanced topics. dotConnect for SQLite includes a help section dedicated to the LinqConnect technology. You can access it online athttp://www.devart.com/linqconnect/docs/.

LinqConnect is developed closely to the Microsoft's implementation of LINQ to SQL,so you might find some useful information in MSDN:

LINQ to SQL
LINQ to SQL: .NET Language-Integrated Query for Relational Data

For hands-on experience use samples shipped with dotConnect for SQLite. You can access the samples from the Start menu.

To understand deeper the works of LinqConnect engine you can watch the generated SQL statements indbMonitoror using the DataContext.Log property.

（编辑：李大同）

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https://www.devart.com/dotconnect/sqlite/articles/tutorial_linq.html

LINQ to SQLite Tutorial

This tutorial guides you through the process of creating a simple application powered byLinqConnecttechnology. In less than 5 minutes you will have a ready-to-use data access layer for your business objects.

In this walkthrough:

Introducing the LinqConnect (LINQ to SQLite) Technology
Requirements
Preparing the Project
Generating Model from Database
Querying Data
Inserting New Data
Updating Data
Deleting Data
Additional Information

Introducing the LinqConnect (LINQ to SQLite) Technology

LinqConnect (formerly known as LINQ to SQLite) is the fast and lightweight ORM solution,which is closely compatible to Microsoft LINQ to SQL and contains its own advanced features,such as complex type support,advanced data fetching options,configurable compiled query caching,and others.

LINQ stands for Language-Integrated Query,which means that data retrieval is no longer a separate language. The LINQ engine allows .NET applications to connect to databases without bothering much about columns and rows. The data you receive is automatically formed as objects ready to use by your business logic.

LINQ to Relational Data may be thought of as an object-relational mapping (ORM) tool. The type-safe LINQ queries get compiled into MSIL on the fly,and the query clauses are translated into SQL and sent to SQLite database for execution. This makes your data access layer safer,faster,and greatly more convenient to design.

Requirements

In this tutorial it is assumed that you already have the database objects created. You have to execute a script from the following file installed by default to
Program FilesDevartdotConnectSQLiteSamplescrm_demo.sql

Preparing the Project

Create a new console application in Visual Studio. It could be any other project type as well,but for simplicity's sake we'll use console project throughout the tutorial. The rest of the tutorial assumes that the name of the project isConsoleApplication1. If you project is named otherwise,you will have to substitute this name with the actual one in Solution Explorer.

Generating Model from Database

Add Devart LinqConnect Model to the project. To do this,right-click on the project node in Solution Explorer,point toAdd,clickNew Item.... In theAdd New Itemdialog selectDatacategory,chooseDevart LinqConnect Modeltemplate,and clickAdd. This automatically launches Create New Model wizard,which creates a new empty model or generates it from database.
ClickNexton the welcome screen.
Fill in connection settings and clickNext.
Choose database objects that will be used in the model. These are all objects from the crm_demo script,including auxiliary tables. ClickNext.
On the next screen you can adjust naming rules for entities and their members. For the CRM Demo database no rules are required,so just clickNext.
InputCrmDemoContextas namespace,andCrmDemoDataContextas the name of DataContext descendant. This will be the name of the main data access class. ClickNext.
PressFinish. The model will be generated and opened in Entity Developer.
In the main menu,clickFile|Save. This updates the generatedCrmDemoDataContextmodel code in Visual Studio.

The model you've just generated is ready to use.

Entity Developer creates classes for all selected tables that represent entities. It also creates a descendant ofDevart.Data.Linq.DataContextclass,which controls the connection to the database,and the whole data flow. This class includes properties and methods named after your database objects. You will use these members to retrieve and modify data in the context. The generated code is contained in the file DataContext1.Designer.cs (DataContext1.Designer.vb). You may write your own partial classes and methods for it in the file DataContext1.cs (DataContext1.vb).

Querying Data

All LINQ to SQLite operations are executed through the DataContext descendant,which is namedCrmDemoDataContextin this tutorial. To retrieve data you have to first create an instance of the context,then prepare a query withLinqConnect,and then access the object returned by the query,which may be a collection of objects or a single object.

Let's read all the data from the table Company,sort it by CompanyID,and output some columns. Add the following block of code to the method Main: