Owners of single entity and owners of group of entities. Constraints or relationship?

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Easy model. Three simple main entities: Task, Owner, TasksGroup.

  • Many Owners might have many Tasks and opposite (many-to-many).
  • Task can optionally belong to only one TasksGroup, but TasksGroup might have many Tasks(one-to-many)

And now the tricky part for me. Owners of each Task within TasksGroup must be the same. There would be also possibility to change Owners of all Tasks within TasksGroup, or of single Task if it doesn't belong to any TasksGroup.

Should there be check constraints? Or maybe there should be "master" relationship established between TasksGroup and Owners? What about relation between Task and Owners if Task belongs to group? What is the good way to design this?

A common strategy in this kind of model is to simplify it by requiring each Task to belong to a TaskGroup. This enables you to remove the direct relationship between a Task and an Owner, at the cost of having single-Task TaskGroups to for stand-alone Tasks.

Fuzzy Logic in Data Modeling: Semantics, Constraints, and Database , Semantics, Constraints, and Database Design Guoqing Chen. 2.2.3. Relationships A relationship is an association among entities. Additionally, if a relationship type relates a weak entity type to its owner type, then this and the role of subordinate (group member) played by the single participating entity type Faculty. Let's see how the particulars of ownership change depending on the number of owners. Ownership in severalty is a legal definition meaning the owner of a property is a single person or legal entity.

Chapter 8 The Entity Relationship Data Model – Database Design , An important constraint on an entity is the key. The key is an attribute or a group of attributes whose values can be used to uniquely identify an individual entity in​  A relationship type R among n entity types E1, E2, , En defines a set of associations—or a relationship set—among entities from these entity types. As for the case of entity types and entity sets, a relationship type and its corresponding relationship set are customarily referred to by the same name, R.

I would go with having RequiredOwners of sorts on the TaskGroup. In fact, one may even have different types of TaskGroup instances where one type requires assigned tasks to have the corresponding owners where others may have different rules. However, that depends on the domain so if this is a real domain you are referring to you would know :)

But it really would be the easiest implementation by have the task group have a list of owners. Well, the TaskGroup would contain either the Ids or a list of value objects representing the owners. Managing the owner assignments on tasks may even be driven from the task group once it is configured.

Bank Consolidation, Efficiency, and Profitability in Italy, Cooperative ownership restrictions also made it virtually impossible to attract BCCs will maintain full autonomy in their relationship with end customers, but there on each foundations' exposure towards any single entity or group of entities. Relationship is nothing but an association among two or more entities; A weak entity is a type of entity which doesn't have its key attribute; It is a single-valued property of either an entity-type or a relationship-type; It helps you to defines the numerical attributes of the relationship between two entities or entity sets

You can do it with a simple schema plus a general database constraint, if your DBMS supports those (and the resulting performance is tolerable). Most don't, unfortunately.

Apart from that, the cleanest approach is overlapping foreign (super)keys. Here is a pseudocode relational approach that doesn't require NULLs; data types and uninteresting attributes omitted:

create table Task {
  TaskId,
  key { TaskId }
};

create table Owner {
  OwnerId,
  key { OwnerId }
};

create table TaskOwner { /* Bog standard association table */
  TaskId,
  OwnerId,
  key { Taskid, OwnerId }
  reference TO_T { TaskId } references Task { TaskId },
  reference TO_O { OwnerId } references Owner { OwnerId }
};

create table TaskGroup {
  GroupId,
  OwnerId,
  key { GroupId }, /* Each group has exactly one owner */
  reference TG_O { OwnerId } references Owner { OwnerId }
};

create table TaskGroupTask {
  TaskId,
  GroupId,
  OwnerId,
  key { TaskId }, /* Each task may belong to at most one group */
  reference TGT_TO { TaskId, OwnerId } references TaskOwner { TaskId, OwnerId },
  /* Foreign superkey coming up */
  reference TGT_TG { GroupId, OwnerId } references TaskGroup { Group, OwnerId }
};

The clue is that TaskGroupTask.Ownerid does double duty as part of the references to both TaskOwner and TaskGroup, and thereby ensures that the owner of the group is identical to the owner of each task in it. There is a foreign superkey here—TGT_TG refers to a superset of TaskGroup's key—but if your DBMS objects to this, a possible workaround is to create a (superfluous) unique index on the superset.

Having a separate TaskGroupTask table instead of a nullable TaskOwner.GroupId attribute may on first glance seem like overkill, but it allows you to use a simple key constraint to enforce the rule that a task may belong to at most one group. Plus it avoids NULLs.

There is a functional dependency { GroupId } -> { OwnerId } in TaskGroupTask that violates Boyce-Codd normal form, but the redundancy is controlled by the TGT_TGreference, and it is necessary to enforce the rules of the situation.

Changing the owner of a group (and all its tasks) involves updating three tables in the same statement/transaction, but a proper RDBMS should have no problem with this.

This design does allow a task to belong to multiple owners and be in one group at the same time, creating a situation where all tasks in a group belong to the same owner, but they belong to other owners (though not groups) as well. It is unclear from your question if this is a problem, so let me know if it is. The design also disallows groups without owners, but I assume this is ok.

Concepts in Federal Taxation 2016, Although individuals are the largest group of taxpaying entities (discussed in Chapter 8) and of events during an entity's life cycle from the viewpoints of sole proprietors, It then reviews each entity in relation to these factors and compares the form of ownership because a partnership must have more than one owner. An Entity Relationship (ER) Diagram is a type of flowchart that illustrates how “entities” such as people, objects or concepts relate to each other within a system. ER Diagrams are most often used to design or debug relational databases in the fields of software engineering, business information systems, education and research.

Concepts in Federal Taxation 2015, Although individuals are the largest group of taxpaying entities (discussed in Chapter 8) and of events during an entity's life cycle from the viewpoints of sole proprietors, It then reviews each entity in relation to these factors and compares the form of ownership because a partnership must have more than one owner. Business entities can own land, hire employees, and are generally considered separate from their owners. In some cases, these legally established businesses are disregarded for tax purposes. There can be benefits and disadvantages to owning a disregarded entity.

Types of Business Ownership, It is not a legal entity that separates the owner from the business, meaning that the owner For a limited partnership, at least one of the partners has a limited liability, Unfortunately, this form of ownership is usually reserved for a group of the increased level of government regulations and the restrictions on the number  An entity set may have more than one candidate key. Primary Key − A primary key is one of the candidate keys chosen by the database designer to uniquely identify the entity set. Relationship. The association among entities is called a relationship. For example, an employee works_at a department, a student enrolls in a course. Here, Works_at

Choose a business structure, Sole proprietorships do not produce a separate business entity. Limited partnerships have only one general partner with unlimited liability, and all Partnerships can be a good choice for businesses with multiple owners, These shed many formalities that typically govern corporations and apply to smaller companies. Relationship instances = a single relationship between two different entities (one-to-one) Ex: relationship instance = works_for the two entities are employee and department, where each employee is associated to a single department (one-to-one).

Comments
  • Thanks David, that's an interesting approach. But I'm afraid in my scenario it would complicate distinguishing between tasks that belong to group, and the ones that are standalone. There can be case when actually "real" group has only one task.
  • In EF Core you could have one FK Task -> TaskGroup on (TaskGroupID,OwnerId), and a separate one Task -> Owner on (OwnerID). But that has its own complexities.
  • This design seems to allow multiple groups per task, and multiple owners per group.