07 Retrieving Objects

Retrieving Objects Strategies for solving the “n+1 selects” problem

Retrieving objects ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Retrieval by identifier ,[object Object],[object Object],[object Object],[object Object],User user = (User) session.get(User.class, userID); User user = (User) session.load(User.class, userID);

Hibernate Fetching Strategies ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Metadata fetching strategies for single-ended associations ,[object Object],[object Object],[object Object],[object Object],[object Object],<class name="Item" lazy="true" > or <class name="Item" proxy="ItemInterface" > <class name="Bid"> <many-to-one name="item" class="Item" outer-join="true" > <class name="Item" lazy="true" batch-size="9" >

Metadata fetching strategies for collections ,[object Object],[object Object],[object Object],[object Object],[object Object],<set name="items" outer-join="true" > <key column="CATEGORY_ID"/> <many-to-many column="ITEM_ID" outer-join="true" class="Item"/> </set> <set name="bids" lazy="true" > <key column="ITEM_ID"/> <one-to-many class="Bid"/> </set> or <set name="bids" outer-join="true" > <key column="ITEM_ID"/> <one-to-many class="Bid"/> </set>

Initializing lazy associations ,[object Object],[object Object],[object Object],[object Object],Category cat = (Category) session.get(Category.class, id); Hibernate.initialize( cat.getItems() );

The n+1 selects problem ,[object Object],[object Object],[object Object],Iterator items = session.createCriteria(Item.class) .add( Expression.eq("item.seller", user) ) .list() .iterator(); List maxAmounts = new ArrayList(); for ( ; items.hasNext(); ) { Item item = (Item) items.next(); float maxAmount = 0.0f; for ( Iterator bids = item.getBids() .iterator(); bids.hasNext(); ) { Bid bid = (Bid) bids.next(); if ( bid.getAmount() > maxAmount ) maxAmount = bid.getAmount(); } maxAmounts.add( new MaxAmount( item.getId(), maxAmount ) ); }

Solving the n+1 selects problem ,[object Object],[object Object],[object Object],List results = session.createCriteria(Item.class) .add( Expression.eq("item.seller", user) ) .setFetchMode("bids", FetchMode.EAGER) .list(); Iterator items = new HashSet( results ) .iterator(); // make results distinct <set name="bids" lazy="true" inverse="true" batch-size="10" > <set name="bids" lazy="true" inverse="true" outer-join="true" > List results = session.createQuery( select i from Item i fetch join i.bids where i.seller = :user ).list(); Iterator items = new HashSet( results ) .iterator(); // make results distinct

Solving the n+1 selects problem ,[object Object],[object Object],Query categoryByName = session.createQuery("from Category c where c.name like :name"); categoryByName.setString("name", categoryNamePattern); List categories = categoryByName. list() ; // SQL: // select ID, NAME, PARENT from CATEGORY where NAME like ?

07 Retrieving Objects

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07 Retrieving Objects