Add "append" to loadEntity

[mikechabot]

Currently, loadEntity overwrites the entity in the store on subsequent calls. This was by design and meant to be a simple implementation. But without added complexity, loadEntity could accept an append argument (boolean), which would optionally append new data from the call to the entity in the store.

[mikechabot]

loadEntity now accepts an options argument that can contain a property named append; it defaults to false, which matches the existing behavior of redux-entity, which is, all calls to an implementation of loadEntity (such as fetchFoo below) overwrite whatever exists in the entity's data property.

However if append is true, subsequent calls to fetchFoo will combine the result of the promise with the existing data in the entity.

Objects

If the result of your data promise is an object, redux-entity creates an array, and pushes the object onto it - it will NOT merge objects.

For example, the promise below returns an object literal. With append set to true and fetchFoo invoked twice, we can see two randomNum objects in the state's data array:

function fetchFoo () {
    return loadEntity(
        'foobar',
        Promise.resolve({ randomNum: Math.random() }),
        { append: true }
    );
}

fetchFoo()(dispatch);  // Call once
fetchFoo()(dispatch);  // Call twice

Final state

{
   "foobar":{
      "isFetching":false,
      "lastUpdated":1480807317751,
      "data":[
         {
            "randomNum":0.99437688223453
         },
         {
            "randomNum":0.02760231206535746
         }
      ],
      "error":null
   }
}

Arrays

If your promise returns an array, redux-entity concatenates it with the existing data on the entity.

For example, the promise below returns an array with two random numbers. With append set to true and fetchFoo invoked twice, we can see four (4) random numbers in the state's data array:

// my custom entity 
function fetchFoo () {
    return loadEntity(
        'foobar',
        Promise.resolve([Math.random(), Math.random()]),
        { append: true }
    );
}

fetchFoo()(dispatch);  // Call once
fetchFoo()(dispatch);  // Call twice

State

{
   "foobar":{
      "isFetching":false,
      "lastUpdated":1480807969474,
      "data":[
         0.3740764599842803,
         0.46176137669506945,
         0.9866826383932925,
         0.7325032000955436
      ],
      "error":null
   }
}