Remove identifier from association_identity

[kerrizor]

By removing the identifier, we're removing duplicates but increasing the weight.

Taking a look at the rake examples output for the "spree" application, we see that the Address model has:

has_many :billing_checkouts, :foreign_key => "bill_address_id", :class_name => "Checkout"
has_many :shipping_checkouts, :foreign_key => "ship_address_id", :class_name => "Checkout"

which, <master> generates (in output/spree.dot):

m_Address -> m_Checkout [arrowhead = "normal", arrowtail = "none", weight = "2"];
m_Address -> m_Checkout [arrowhead = "normal", arrowtail = "none", weight = "2"];

With this change, our output DOT shows this as:

m_Address -> m_Checkout [arrowhead = "normal", arrowtail = "none", weight = "4"];

I'm a bit nervous about this change; what I think I might prefer is to see an option for labelling relationships, either all of them, or when there are multiples like this that have different :foreign_key declarations, but it isn't a huge priority, something for a future release. For now, this solves the problems reported by many, many users.

Rebase of #296

See #70 (comment)

Fixes #278

(h/t to @afn who submitted the PR originally)

[magneticnorth]

Hi - what is the graphviz bug causing the problem? Maybe we fixed it. There was a lot of work on handling multiple edges, but the bookkeeping for arrowheads and directions was not well thought out and there could still be problems. Some parts of the code really need an overhaul. Sorry for the difficulties. north from graphviz org

[pedros007]

@magneticnorth #196 (comment) might shed some light. rails-erd would invoke GraphViz, which seg faulted. I narrowed it down to rails-erd would generate a dotfile with concentrate = "true";. When I removed that, GraphViz would no longer segfault.

[kerrizor]

@magneticnorth thank you for all the work on Graphviz!

One of my ToDos the next few weeks is to set up a series of repos that we can reproduce bugs from; if we can reproduce issues like this segfault, I can discover if the issue is our DOT generation, or something in specific versions of Graphviz; we don't really enforce or recommend any particular versions, but perhaps we should start...