Data inconsistency prohibition if a table refers to another via two many-to-many relationships

Question

I have following database design (by => foreign key constraints are depicted):

Company[id]
CompanyRealm[id, company_id=>Company.id]many2many

Project[id, company_id=>Company.id]
ProjectRealm[id, project_id=>Project.id, company_realm_id=>CompanyRealm.id]many2many

Problem is that provided database design allows inconsistent data. For example:

Company1(id=1)
Company2(id=2)
CompanyRealm(id=11, company_id=1)
Project(id=33, company_id=2)
ProjectRealm(id=44, project_id=33, company_realm_id=11)

(Company can have many Realms, Project belong to Company and may concern any number of company's Realms)

ProjectRealm refers to two different companies:

• Company1 (via CompanyRealm); and
• Company2 (via Project).

Is something wrong with my DB design?
If yes - which prescriptions are violated?
If no - how to prevent incorrect data insertion (via constraint? trigger with exception?)

Answer 1

You are right, the design allows inconsistencies, exactly what you notice. A ProjectRealm may be referring through Project to a company and through CompanyRealm to another company.

This is not uncommon, it appears when there is a triangular or a "diamond" shape in the relationships:

Realm          Company
   \           /     \
    \         /       \
     \       /         \
    CompanyRealm     Project
             \         /
              \       /  
               \     /    
             ProjectRealm

The common solution, if you want to enforce this via DRI (Declarative Referential Integrity), ie. through usual UNIQUE, FOREIGN KEY and CHECK constraints - and not though triggers - is to use composite keys for the ProjectRealm REFERENCES CompanyRealm and for the ProjectRealm REFERENCES Project foreign keys.

You will need to replace the company_realm_id with 2 columns (company_id, realm_id) and add a composite UNIQUE constraint on each of the two referenced tables (or modify their PRIMARY KEY):

Company [id]

Realm [id]

CompanyRealm [id, realm_id=>Realm.id, company_id=>Company.id,
              UNIQUE(company_id, realm_id)]

Project [id, company_id=>Company.id,
         UNIQUE(company_id, id)]

ProjectRealm [id, project_id, company_id, realm_id,
              (company_id, project_id) => Project (company_id, id),
              (company_id, realm_id) => CompanyRealm (company_id, realm_id)]

See also several other questions, where the same (or quite similar) problem appears. The diamond pattern is obvious in all:

• Best data modelling approach to handle redundant foreign keys in relational model
  and the nice answer and logical models and diagrams by MDCCL.
• Foreign Key Constraint over two tables
• How to represent it with UNIQUE indexes and foreign keys?
• Many to Many and Weak Entities
• Constrains relationship