Crafting NFT metadata
Turn table data into JSON objects and array for ERC721 compliant metadata.
Tableland offers a dynamic and flexible way to store NFT metadata. You can use Tableland for structuring the top-level metadata and nested attributes in tables and compose them together using a read query. In order to craft the data into ERC721 compliant metadata, developers can store NFT attributes (traits like integers or strings) and pointers large media on IPFS (or similar) in table cells—with access controls to allow for data mutability.
Ultimately, you'll need to form a JSON object with ERC721 compliance for each row in the table. Here, let's ignore some of the static top-level data and focus on dynamic attributes where
N represents the token's
"name": "Token #N",
For these examples, let's assume there exists
attributes where every
id represents an NFT and multiple rows can have the same
id. Its schema is the following:
It has data that defines its attributes, and the top-level data (
external_url) can be composed within the
For example, the first row represents a token with an
1 and some metadata that defines its
color trait as the string
blue, the second is a token with an
2 and a
To serve NFT metadata from a row in a table, you need to write a SQL statement that will:
- Extract the right row given a specific
id(which should match with the NFT's
- Convert the row into ERC721 compliant JSON metadata.
You’ll want to query
attributes above with a
WHERE clause on the
id column—for example, the row where the
1. With SQL functions, you can use the json_object() function to turn table data into a JSON object. Check out the JSON functions documentation for more details.
The TL;DR is that you can define a key-value pair in the function itself but as a set of comma separated values. The first value is the key (a string), the second value is the table's column name, and so on and so forth. At the end of the query, the
<id> value represents the value that you would pass here; in our example table, the available
ids is either a
id = <id>;
This will create an object for only the rows where, for example,
id = 1 since each
id represents a single NFT, and this pattern can be repeated for each and every token / row. However, the example above only a component of composing the entire metadata object.
JSON array with objects
NFT metadata has top-level keys of
external_url, which often may incorporate the
id. There is also an
attributes key that is an array of objects which match the JSON object exemplified above. Note that other key-values are possible in NFT metadata. For simplicity sake, these are ignored for the moment but could easily be incorporated within the query itself (e.g., a top-level
description that's the same for every NFT).
You'll need to create an object that contains these top-level definitions, and the
attributes will need to make use of the JSON group array function. Namely, the
json_group_array() will allow you to take one or more table rows that meet the
WHERE clause condition and create JSON objects from them.
Here, you're using both the JSON object and array functions along with the
|| operator to concatenate values into strings. Note that if you're not using IPFS to store images (e.g., if storing on a hosted server), you should replace
'ipfs://QmVK.../' with your base URL.
'name', 'Token #' || id,
'image', 'ipfs://QmVK.../' || id,
'external_url', 'https://example.com/' || id,
WHERE id = <id>;
Without going into too much detail on IPFS, this assumes the
image is being stored in some directory where the path to the image can simply be appended to the end of the URL (e.g.,
ipfs://QmVK.../1 would point to an image for an NFT
1). Hence, when someone views the underlying value mapped to the
image key, it'll be the location of the actual media. The same pattern is often used for the
external_url to point to some website landing page for the NFT.
If you queried for the row where
id = 1 in
attributes, the result would look resemble the following:
"name": "Token #1",
Because the JSON array function aggregates results, it will automatically include new rows that use the same
id due to the way the query is structured. For example, assume there are now two rows for a token:
If you run the same query above, it'll produce an
attributes array with two objects instead of just one:
"name": "Token #1",
To make the metadata change, all it takes is an
UPDATE statement. Perhaps the owner has permission to change the
color of their owned token. You would then execute a statement like so:
color = <color>
id = <id>;
For example, the owner of token ID
1 might want to set the color to
purple—using these values in the query above would change the table's data for that row and column, thus, automatically returning the new
purple attribute whenever that query is made.
In other words, once you set a read query in place, it will always pull the live table state and reflect its latest changes.
More than one table
In the preceding examples, some "shared" metadata was composed with values defined within the read query, including the (truncated) IPFS CID
external_url. Instead, these shared values could exist in a "lookups" table and then composed with SQL.
If you want to change existing metadata, all it really takes is mutating table values. Thus, rather than the read query defining hard-coded values that could change (such as updating the folder that's storing images and changing the
QmVK... CID), you can future-proof any changes by storing shared values in another table. In other words, the read query would handle composing data across multiple tables that define potentially mutable commonalities.
A new table can be created—call it
lookups—with the following schema.
The table is pretty small (one total row) but could have more values included, based on your metadata needs.
This holds data that is now part the top-level data (
external_url) and can be composed within the
SELECT statement. Note that if you're not using IPFS to store images, you can replace the
cid column so that it mimics the
external_base_url setup. This would slightly alter the query portion for
image and resemble how the
external_url is formed.
'name', 'Token #' || id,
'image', 'ipfs://' || cid || '/' || id,
'external_url', external_base_url || id,
WHERE id = <id> GROUP BY <id>;
The JSON response is the same as before but much more future-proof and ready for dynamism. You could also choose to add other columns, such as an
animation_base_url or similar.
Looking for more? Check out the page on how to build an NFT, including additional resources including building a dynamic NFT in Solidity.