one-to-one | 易学教程

Linq 2 SQL One to Zero or One relationship possible?

阅读更多关于 Linq 2 SQL One to Zero or One relationship possible?

问题 Is it possible to create a one to zero or one relationship in Linq2SQL? My understanding is that to create a one to one relationship you create a FK relationship on the PK of each table. But you cannot make the PK nullable, so I don't see how to make a one to zero or one relationship work? I'm using the designer to automatically create the model - so I would like to know how to set up the SQL tables to induce the relationship - not some custom ORM code. 回答1: You're partially correct...but

Django - one-to-one modelAdmin

阅读更多关于 Django - one-to-one modelAdmin

I am moderately proficient with django and trying to use model forms for an intranet project. Essentially, I have a "Resources" model, which is populated by another team. Second model is "Intake", where users submit request for a resource. It has one-to-one mapping to resource. Objective is to only allow 1 resource allocation per intake. Now, Intake model form shows the form, with a drop-down field to resource, with a caveat that it shows all resources, regardless of previous allocation or not. ex. if resource is taken by an intake, save button detects that disallow saves. This is expected,

JPA unidirectional @OneToOne vs @ManyToOne with Hibernate - no difference?

阅读更多关于 JPA unidirectional @OneToOne vs @ManyToOne with Hibernate - no difference?

问题 According to book Pro JPA 2 the main difference between unidirectional @ManyToOne and @OneToOne is that in @OneToOne: Only one instance of the source entity can refer to the same target entity instance. In other words, the target entity instance is not shared among the source entity instances. In the database, this equates to having a uniqueness constraint on the source foreign key column (that is, the foreign key column in the source entity table). The thing is, when I create such a mapping

Sequelize one to one relation

阅读更多关于 Sequelize one to one relation

I have two models, Video and Frame. Frame.belongsTo(models.Video); Video.hasMany(models.Frame, { as: "Frames" }); I now need a way to specify first and last frame for a video, but can't get it to work. I tried this: Video.hasOne(Frame, { as: "FirstFrame" }); Video.hasOne(Frame, { as: "LastFrame" }); but that creates FirstFrameId and LastFrameId in the Frames table instead of the Videos table. I need to have video.get/setFirstFrame() and video.get/setLastFrame() functions available. How can I do this? Ellebkey You don't need to set belongTo and hasMany as you show at first. Use only one

SQLAlchemy query filter on child attribute

阅读更多关于 SQLAlchemy query filter on child attribute

问题 My model consists of a Parent and Child with a one-to-one relationship: class Parent(Base): __tablename__ = 'parent' id = Column(Integer, primary_key=True) name = Column(String) child = relationship("Child", backref="parent", uselist=False, lazy='joined') class Child(Base): __tablename__ = 'child' child_id = Column(Integer, ForeignKey(Parent.id), primary_key=True) value = Column(Integer) my test data are the following: q = s.query(Parent) pd.read_sql(q.statement,s.bind) id name child_id value

Hibernate code…exception in onetoone mapping

阅读更多关于 Hibernate code…exception in onetoone mapping

I am getting weired exception and not able to trace why it so. Please help me here. I am just using OneToOne mapping with PrimarykeyJoinColumn property. @Entity @Table(name="mediashow_user1") public class UserVO implements Serializable{ private static final long serialVersionUID = 6066636545309839156L; @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long empid ; private String email = null; private String fname = null ; private String lname = null ; private String mname = null ; private String uname = null ; private String passwd = null ; private String serctquestion = null ;

In SQL / MySQL, are there reasons not to put one-to-one relationship in the same table?

阅读更多关于 In SQL / MySQL, are there reasons not to put one-to-one relationship in the same table?

One-to-one relationship could usually be stored in the same table. Are there reasons not to store them in the same table? Number and type of columns. There is a limit on the size of the columns in a table. See here . There is a maximum of 8,060 bytes per row. Very large tables can also affect performance and can be difficult to optimize and index well. This is apart from keeping data the is conceptually different, apart from each other. For example, a country and currency have a 1 to 1 relationship (illustrative example, I know this is not always the case). I would still not keep them together

Hibernate: @ManyToOne(fetch = FetchType.LAZY) does not work on non-primary key referenced column

阅读更多关于 Hibernate: @ManyToOne(fetch = FetchType.LAZY) does not work on non-primary key referenced column

I have 2 tables: Order [OrderId(PK), OrderShipmentCode, ...] and Shipment[ShipmentId(PK), ShipmentCode, ...] . In Order class, I declared shipment field as follows: @ManyToOne(fetch = FetchType.LAZY) @JoinColumn(name = "OrderShipmentCode", referencedColumnName = "ShipmentCode", insertable = false, updatable = false, nullable = false) private Shipment shipment; When I get the list of Order , the Shipment is also loaded (I saw many separate SELECT queries). But I want the Shipment to be lazy loaded, not to be fetched together with Order . For other tables, if the referenced column is primary key

How to lazy load a one-to-one composition via hql

阅读更多关于 How to lazy load a one-to-one composition via hql

If have an entity A with a bidirectional one-or-zero-to-one mapping with entity B. The mapping is as follows: <class name="EntityA" table="TABLE_A" mutable="true" lazy="true"> <id name="idA" type="long" column="pk_a" unsaved-value="null"> <generator class="sequence"> <param name="sequence">pk_a_seq</param> </generator> </id> <one-to-one name="propertyB" class="EntityB" property-ref="propertyA" constrained="true" outer-join="false"/> </class> and <class name="EntityB" table="TABLE_B" mutable="true" lazy="true"> <id name="idB" type="long" column="pk_b" unsaved-value="null"> <generator class=

@OneToOne getting returned as ManyToOneType

阅读更多关于 @OneToOne getting returned as ManyToOneType

I have the following POJO: public class SampleBean1 { @Id @GeneratedValue(generator = "system-uuid") @GenericGenerator(name = "system-uuid", strategy = "uuid") protected String id; @OneToOne(cascade=CascadeType.ALL) @JoinColumn(name="OneToOneID") protected SampleBean1 oneToOne; @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.LAZY) @JoinColumn(name="OneToManyID") protected List<SampleBean1> oneToMany; @ManyToOne(cascade=CascadeType.ALL) @JoinColumn(name="ManyToOneID") protected SampleBean1 manyToOne; @ManyToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER) @JoinTable(name=