Dynamic Join Assoc Join LoadOpt Deferred. LINQ-to-SQL Part 2. Radu Nicolescu Department of Computer Science University of Auckland.

Transcription

1 LINQ-to-SQL Part 2 Radu Nicolescu Department of Computer Science University of Auckland 7 October / 22

2 1 Dynamic LINQ 2 Inner joins 3 Associations 4 Joins by navigation 5 Load options 6 Deferred evaluation 2 / 22

3 Dynamic LINQ More flexible, string based format for properties... Very useful for complex sorting: OrderBy / OrderByDescending ThenBy / ThenByDescending ThenBy / ThenByDescending... One single OrderBy can cover all the above combinations! 4 / 22

4 Dynamic LINQ Equivalent: 1 v ar w1 = MyOrders 2. Where ( o => o. P r i c e > 2 0 ) ; 1 v ar w2 = MyOrders 2. Where ( P r i c e > 20 ) ; Equivalent: 1 v ar a1 = MyOrders 2. OrderBy ( o => o. P r i c e ) 3. ThenBy ( o => o. OrderID ) ; 1 v ar a2 = MyOrders 2. OrderBy ( P r i c e, OrderID ) ; 5 / 22

5 Dynamic LINQ Equivalent: 1 v ar d1 = MyOrders 2. OrderByDescending ( o => o. P r i c e ) 3. ThenByDescending ( o => o. OrderID ) ; 1 v ar d2 = MyOrders 2. OrderBy ( P r i c e DESC, OrderID DESC ) ; Think about SortingCriteria string in A / 22

6 Explicit Inner Join fluent style Left table, right table, left key (lambda), right key (lambda), final projection 1 var i n n e r j o i n = 2 nwd. MyCustomers 3. J o i n ( 4 nwd. MyOrders, 5 c => c. CustomerID, 6 o => o. CustomerID, 7 ( c, o ) => new { 8 c. Name, 9 c. Age, 10 o. D e s c r i p t i o n, 11 o. P r i c e 12 } ) ; 8 / 22

7 Explicit Inner Join query style Left table, right table, left key (disguised lambda), right key (disguised lambda), final projection 1 var i n n e r j o i n = 2 from c i n nwd. MyCustomers 3 j o i n o i n nwd. MyOrders 4 on c. CustomerID equals o. CustomerID 5 s e l e c t new { 6 c. Name, 7 c. Age, 8 o. D e s c r i p t i o n, 9 o. P r i c e 10 } ; 9 / 22

8 Associations Enhance the model to expose the PK/FK association between MyCustomers and MyOrders Each MyCustomer may have any number ( 0) of MyOrder s Each MyOrder belongs to one MyCustomer or none as we didn t enforce this constraint in the database definition! Tools expose such associations automatically... Here we create them by code, via additional model properties and meta-model attributes 11 / 22

9 MyCustomers MyOrders MyCustomers PK 1 n MyOrders FK MyOrders is (i) an SQL table name and (ii) a pointer to the set of associated orders 1 [ Table (Name= MyCustomers ) ] 2 p u b l i c c l a s s MyCustomer { p r i v a t e EntitySet<MyOrder> MyOrders ; 5 6 [ A s s o c i a t i o n ( S t o r a g e= MyOrders, 7 ThisKey= CustomerID, OtherKey= CustomerID, 8 D e l e t e R u l e= NO ACTION ) ] 9 p u b l i c EntitySet<MyOrder> MyOrders { 10 get { return MyOrders ; } 11 set { MyOrders. A s s i g n ( v a l u e ) ; } 12 } 13 } 12 / 22

10 MyOrders MyCustomers MyOrders FK n 1 MyCustomers PK MyCustomer is (i) a C# model type and (ii) a pointer to the associated customer 1 [ Table (Name= MyOrders ) ] 2 p u b l i c c l a s s MyOrder { p r i v a t e EntityRef<MyCustomer> MyCustomer ; 5 6 [ A s s o c i a t i o n ( S t o r a g e= MyCustomer, 7 ThisKey= CustomerID, OtherKey= CustomerID, 8 I s F o r e i g n K e y=true ) ] 9 p u b l i c MyCustomer MyCustomer { 10 get { return MyCustomer. E n t i t y ; } 11 set { MyCustomer. E n t i t y = v a l u e ; } 12 } 13 } 13 / 22

11 Joins by Navigation fluent style Join simplified by navigation over associations Generates: (i) INNER JOIN, if DB defines referential constraints, or (ii) LEFT OUTER JOIN, otherwise (our case) 1 v ar MyCustomers = nwd. GetTable <MyCustomer >(); 2 v ar MyOrders = nwd. GetTable <MyOrder >(); 3 4 var n a v j o i n = 5 MyOrders 6. S e l e c t ( o => new { 7 o. MyCustomer. Name, 8 o. MyCustomer. Age, 9 o. D e s c r i p t i o n, 10 o. P r i c e 11 } ) ; 15 / 22

12 Joins by Navigation query style Join simplified by navigation over associations Generates: (i) INNER JOIN, if DB defines referential constraints, or (ii) LEFT OUTER JOIN, otherwise (our case) 1 v ar MyCustomers = nwd. GetTable <MyCustomer >(); 2 v ar MyOrders = nwd. GetTable <MyOrder >(); 3 4 var n a v j o i n = 5 from o i n MyOrders 6 s e l e c t new { 7 o. MyCustomer. Name, 8 o. MyCustomer. Age, 9 o. D e s c r i p t i o n, 10 o. P r i c e 11 } ; 16 / 22

13 Load options Assuming N customers, how many SQL queries are needed for this conceptually outer join code? 1 var query = 2 from c i n MyCustomers 3 where c. Age > 40 4 s e l e c t c ; 5 foreach ( var c i n query) { // 1! 6 Console. W r i t e L i n e ( {0} {1}, c. Name, c. Age ) ; 7 foreach ( var o i n c.myorders) { // N? 8 Console. W r i t e L i n e ( \ t {0} {1}, 9 o. D e s c r i p t i o n, o. P r i c e ) ; 10 } 11 Console. W r i t e L i n e ( ) ; 12 } Chatty interface: 1 + N SQL queries = 1 for query1 plus one for each customer c 18 / 22

14 Load options Chunky interface: just 1 big SQL query (based on a LEFT OUTER JOIN) 1 DataLoadOptions d l o = new DataLoadOptions ( ) ; 2 d l o. LoadWith<MyCustomer >( c => c. MyOrders ) ; 3 nwd. LoadOptions = d l o ; 4 5 var query =... 6 foreach ( var c i n query) { // 1! 7 foreach ( var o i n c.myorders) { // 0!! 8 } 9 } Chunky is often better than Chatty: faster, less traffic / 22

15 Deferred evaluation SQL queries are built in memory and sent to the database in the last moment, when their result is really needed 1 var query1 = MyCustomers. Where ( c => c. Age < 5 0 ) ; 2 // nothing is sent yet! 3 4 var query2 = query1. Take ( 1 ) ; 5 // query2 builds a bigger SQL on top of query1 6 7 query2.dump( query2 ) ; // SQL sent for query2! SQL for query2 only NO SQL for query1 1 SELECT TOP ( 1 ) [ t0 ]. 2 FROM [ MyCustomers ] AS [ t0 ] 3 WHERE [ t0 ]. [ Age ] < / 22

16 Deferred evaluation In this scenario, two SQL queries are sent to the database 1 var query1 = MyCustomers. Where ( c => c. Age < 5 0 ) ; 2 query1.dump( query1 ) ; // SQL sent for query1! 3 4 var query2 = query1. Take ( 1 ) ; 5 6 query2.dump( query2 ) ; // SQL sent for query2! SQL for query1 and query2 1 SELECT [ t0 ]. 2 FROM [ MyCustomers ] AS [ t0 ] 3 WHERE [ t0 ]. [ Age ] < SELECT TOP ( 1 ) [ t0 ]. 6 FROM [ MyCustomers ] AS [ t0 ] 7 WHERE [ t0 ]. [ Age ] < / 22