WIP, immutable relationship

[MacondoExpress]

WIP, Immutable relationship (readonly directive applied to relationship field)

Please provide a description of the work completed in this branch.

Issue

Please provide a link to the GitHub issue in which the proposal for this work was discussed below.

It's important that changes are discussed in advance to make sure we choose the right solution and to avoid duplication of effort.

Closes:

Checklist

The following requirements should have been met (depending on the changes in the branch):

• [ ] Documentation has been updated
• [ ] TCK tests have been updated
• [ ] Integration tests have been updated
• [ ] Example applications have been updated
• [ ] New files have copyright header
• [ ] CLA (https://neo4j.com/developer/cla/) has been signed