how to add object to firestore first index of object array

Question

i've been googling around about how to add an object into an array in firestore, and found the arrayUnion() able to add an object into firestore array, but it only add the object into last index of array, but how to add it into first index of array?

//add "greater_virginia" into last index of array
washingtonRef.update({
    regions: firebase.firestore.FieldValue.arrayUnion("greater_virginia")
});

//how to add "greater_virginia" into first index of array?

its basically same as arrayUnion but instead of add it into last index, i want to add it into first index of array.

windowsill · Accepted Answer · 2020-04-15 04:54:47Z

2

If Firestore arrays behave anything like realtime-database arrays, then they don't actually exist. As far as I know, they are store as maps, like:

{
  0: "first element",
  2: "second and so on",
}

You can probably see how an unshift would be a big transformation. In fact, firestore doesn't let you do this, saying "...in order to avoid some of the issues that can arise in a multi-user environment, you'll be adding them with more of a set-like functionality".

With that in mind, this problem is usually solved at the application level by fetching the array, mutating it as needed, then setting the whole thing.

Bit of further reading https://firebase.googleblog.com/2018/08/better-arrays-in-cloud-firestore.html

PS: be careful with the arrayUnion operator, because it actually performs a add to set

answered Apr 15, 2020 at 4:54

windowsill

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4 Comments

Doug Stevenson Over a year ago

Firestore array fields definitely do exist. They have their own dedicated field type that are different than maps. It's not like Realitme Database at all.

Nathan Priyasadie Over a year ago

thank you for the answer very clear answer, do you know any syntax that i can use to fetch it manually, unshift it then push it into firestore? maybe i can do more research on it

windowsill Over a year ago

Thanks for the clarification @DougStevenson. As for how to get and set data from your database, you'll have to read the beginning of the docs firebase.google.com/docs/firestore/query-data/get-data

windowsill Over a year ago

Conceptually though it will be something like doc = get doc from collection; newArray = [newElement, ...doc.arrayField]; set newArray on arrayField of doc;

Doug Stevenson · Accepted Answer · 2020-04-15 05:02:25Z

2

Firestore doesn't offer any way to modify items of array fields by index. arrayUnion will only ever append to the end of the array if the element doesn't already exist.

If you want to modify an array by index, you will have to read the document, modify the array in memory to appear how you want, then write the modified array back to the document.

answered Apr 15, 2020 at 5:02

Doug Stevenson

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2 Comments

Albert Renshaw Over a year ago

Note: Unlike arrayUnion, this will fail atomicity

Doug Stevenson Over a year ago

@AlbertRenshaw You can use a transaction for that.

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