Ensure duplicate responses are not created by chained responses to the same question

[rsutphin]

Since surveyor posts responses asynchronously, it's possible for several changes in a row to result in creating separate response records instead of creating and then updating the same record. E.g., say you have a pick-one question with a free-entry answer and a coded answer. You:

• Select the coded answer
• Change to the free-entry answer
• Enter a value for the free-entry answer and tab away from (blur) the free-entry answer

Surveyor will make an AJAX call after each of those bullets. If all of them happen before the first AJAX call returns with the response ID, the server side code will create separate response records for each. This confuses analysis (which response is the real one?) since it is legal in general for a question to have multiple responses (either when it is a repeated question or a pick-many question). There is also code elsewhere in Surveyor (e.g., #297) which assumes that single-answer questions will only have one response per response set.

Proposed solution:

1. Surveyor now associates a UUID with each response (the api_id column). Pregenerate UUIDs for each possible response when rendering the UI. Dynamically added responses (i.e., for repeaters) will generate the UUIDs on the client side.

2. Submit the UUID with the user's response. Only create a new response record if there isn't one already with the UUID. Use a uniqueness constraint on responses.api_id to ensure that you don't get duplicates. If the constraint is violated during save, retry as an update.

Comments appreciated.

[rsutphin]

@ybushmanova notes that the date picker in recent versions of surveyor is particularly prone to creating duplicate responses.

[rsutphin]

Another vulnerable situation in the current master is a repeater with a single free-text value question. A cucumber @javascript feature which:

• Enters text for the first instance of the repeating question
• Clicks the + add row button
• Enters text for a second repetition
• Blurs the second rep's field

... will consistently create three responses instead of two, duplicating the first one.