Document associations

[jewzaam]

There's a doc that describes the algorithm for associations already but:

1. it needs moved here
2. additional documentation on how to use associations needs added
3. add docs why it isn't called 'join'

For the third point, some things to consider:

• joins in RDBMS are tight couplings, enforceable with RI (foreign keys)
• lightblue associations are loose couplings, you can associate anything to anything else with any query you would like to try with no restrictions

[jewzaam]

moved